You can swap an MPFI system into your DX, and for cheaper than a lot of bolt-ons. The gains will be significant, in fact, its said that the chief difference between the DX and the Si when it comes to making power is not the .1L displacement but the increased input due to the MPFI system.
So getting this show on the road, you'll need the following parts:

From an Si:
MPFI Intake Manifold (intact with fuel rail, injectors, throttle body, etc)
Injector Resistor Box
PM6 ECU from Si
Si Distributor
Both sides of the Si Distributor plug if possible

From any car:
A few small pins from the ECU side B and C plugs on the wiring harness. If you can just cut a whole plug and disassemble it later, do it, because this is a pain.

From the parts store:
Various colour wire
A coolant hose with a 180-degree bend, like the one on the Si.
Intake Manifold Gasket
Coolant
Solder
GOOD Electrical tape. 3M 33+ is what I used.
Wire Shrink Wrap
NO BUTT SPLICES, BAD!

First off, you'll need to release the fuel pressure and drain the radiator so you don't spray gas and coolant all over you when you're unhooking the manifold.

Now to get your manifold off. You'll need to get creative with the swivels and stuff with the ratchet to get to some of the bolts on the bottom of the manifold. Remove the manifold support first, it'll afford you some room under there, and remove any vacuum hoses in the way, remembering where they went by labelling them or using your photographic memory. Remove the throttle cable from the gas pedal by using needle nose pliers to squeeze the tabs together and pull it back off the pedal. After you've removed all the bolts and unplugged everything, pull the manifold back and put it to the side. Good riddance.

Next you'll want to mount the injector resistor box on the screw on the driver side shock tower that the harness clip is fastened to. You can remove the bracket that is already there because the injector resistor box has a bracket on it that you can put it on.

Remove your distributor by loosening the 3 bolts that hold it on, just like you were adjusting the timing but by taking all 3 fully out. Then pull it a little, you may have to turn it some to get it out. Replace this with the Si distributor. If the lugs on the distributor seem to be matching up but it won't go in, rotate them 180 degrees. The lugs are offset slightly to prevent you from installing it 180 degrees off, so the only way it will go in is if it is in the right configuration. Now, with the other side of the Si distributor plug you hopefully got, remove the pins from the DX distributor plug (square) by using a precision screwdriver to release the tabs, and pushing the pin out of the back, and put them in the Si plug so that the colours all match the corresponding colours on the other side. You'll be missing 2 wires, blue with a green stripe and blue with a yellow stripe.

Here's where the wiring begins.

Run a blue wire with a green stripe (or something close) from that plug, through the boot in the firewall above the passenger foot-well down by the ECU. Do the same with a blue wire with a yellow stripe. Connect these 2 wires to the correct locations on the Si distributor plug. You've just added the wiring for the Crank Angle Sensor, which is vital to the synchronistic operation of the MPFI system.

Next, find the green plug that has 3 wires feeding it on the harness that you removed from the ECU. There is only one plug with 3 wires feeding it, so that’s it. This is the Throttle Position Sensor. Since the throttle on the DX goes in the opposite direction as the throttle on the Si, you'll need to switch the 2 outer wires of the TPS (not the middle one). Do a pin swap and you're done. Well... not quite. The throttle body on the Si is also up right where the intake enters the manifold, so you'll need to extend the wires about 10 inches. Use SOLDER, electrical tape, and shrink-wrap. Don't butt-splice things in your engine or you'll end up paying for it one day when you're driving and your ECU starts throwing codes all over the place.

Now for the injector wiring. This is easier than everybody says it is, just take your time and make sure you're using the right wires. When in doubt, check again. When you think you've got it right, double-check it one more time. That being said: you should have some wires connected to each of the injectors. The red wires are all grounds that go to the Injector Resistor Box, and should be connected to a square plug on one end. This plugs right into the box. That was easy. Now you need to run the wiring to the other side of the injectors, and supply power to the injector resistor box. Starting out, connect the yellow wire with the black stripe from the injector resistor box to the large yellow wire with the black stripe that used to feed the main injector of the DPFI system. Next the injectors: Connect the brown wire from Injector #1 to the yellow wire that went to the secondary DPFI injector.

Connect the blue wire from injector #3 to the red wire that went to the primary injector.

Connect the Red wire from injector #2 through the firewall, and leave it by the ECU

Connect the Yellow wire from injector #4 and through the firewall the same as you did with #2.

Now the final step in the wiring is to connect the correct wires to ECU. You should have 3 wires that you put through the firewall: a blue with a green stripe, a blue with a yellow stripe, a red wire, and a yellow wire.

First, remove the pin from C1 and plug it into the open position at B10

Then do the same with C2, moving it to B12

This may be a pain to remove the pins, but have patience. It takes something very small like a tiny precision screwdriver to release the pins, and you need to poke around in there and look inside with a flashlight to figure out where the release is. Once you've finally figured it out and you've moved the pins, you've made room for the crank angle sensor wires (the 2 blue ones). Connect the blue/green wire to pin C1, and the blue/yellow one to pin C2.

2 wires are left, the red one and the yellow one. There are wires coming from pins A3 and A7 (the bigger plug) on the ECU, but they no longer feed anything under the MPFI system. Cut them and try not to think about it too much. Now connect the red wire to pin A3, and the yellow wire to pin A7. This completes the wiring. Don't put your ECU back yet until your car starts, you'll want it out anyway to be able to easily check for codes.

Now that you've double-checked every connection, triple check them, make sure everything is clean, no wires are exposed, and no tape is hanging. Now get the MPFI manifold, slide on the new gasket, (make sure any previous remaining gasket that is stuck to your MPFI manifold has been removed), and slide the manifold on. Don't connect any wires yet, first screw on the manifold tightly, tightening the bolts just like a header, tightening the inside bolts first, and alternating moving out. Reconnect all the vacuum hoses to their correct locations, at this step it helps to have the vacuum diagram available off the bottom of an Si hood, or an Si to look at. Use the 180 degree bend coolant hose in place of the old coolant hose. Connect the throttle cable. Plug in all the sensors as follows: The white plug fed by red/yellow and green/white wires goes to the air temperature sensor on the driver's side of the manifold. The green plug with 3 leads goes to the throttle position sensor on the throttle body. The green plug fed by black/yellow and blue/yellow wires goes to the EACV on the back of the manifold. There is a plug fed by black/yellow and orange wires. This plug just hangs there because you won't need it anymore.

Now double-check everything once again. Refill the radiator, make sure the line to the fuel rail is extra tight, make sure you remembered to connect your cold air intake back to the throttle body, cross your fingers, and turn the key. If all is right, your car might take a little bit to realize that you did a brain transplant, and you may have to adjust the timing and retard it some to get it to start, but once its going, you'll be good. Take it for a spin, watching for the check engine light. In the off chance that you get one, check the little light on the ECU for codes to see what the problem is. But you took your time, and did it right, so you shouldn't have to worry about this. So enjoy what you've been missing out on for so long. I also have the injectors for the si if you would like to check yours after you do all this and still have a problem.

Josh