eh the HG job on that car is not so bad. wile its apart he can take care of a lot of little oddes and ends that should be done under the hood anyhow.

The cheapest and easyist swap is the Red top, the motor can be taken from cofers u pull it for about $100, the stock blue top harness, manifolds, trans ect will all work on the red top.
the diffrence is the redtop has higher compression and smaller intake ports with no Tvis. makeing more power then the blue top across the power band.

the 2nd sorta but not quite easy, and quite a bit more pricy swap would be a 20valve motor from a AE101 or ae111 corolla in japan. the motor bolts in to the mr2, and you use your stock transmission, but you will have to dissasemble the wireing harnss and re form it to work in the diffrent configuration. not an overly hard thing to do but time consuming.

there are two 20valve motors. the silver top and the black top,
the silver being the cheaper ($600-800) the black top($800-1000) has a few minor upgrades over the silver top. both are indevidual trottle body, the black top is map, so you can run the throttle bodys open like a sidedraft carbed car, the sound is simply evil.
the silver is veriable valve timing (vvt) its on a rpm switch like vtec, so it just changs with rpm, but unlike vtec it does not switch from 2 cam profiles. it rotates the intake cam as the rpm changes. the black top is quite a bit more advanced in this area. it has VVTL-i thats veriable valve timing and lift with intellagence. it changes the timing, lift baised on quite a few factors, rpm, trottle position, temp, ect. the black top also comes with a 6speed, that can be retrofitted to work in the mr2. with some mods to the shift linkage. (its ment to be in the front, not the back of the car)
they both make just about 150-160hp the silver being on the lower end of that, black on the higher end. but tuners have pulled over 200hp from both motos. without forced induction.
200hp in a firstgen mr2 will rip.

The most insane swap would be a 3S-GTE. the 2.0L turbo motor from a 2nd gen mr2 or celica, this motor is NOT for autox. where all the all the other motors make good power and dont change the weight of the car at all, this one converts the car to a monster. 600hp is not out of the question, but it would lift the wheels. info on this swap can be filled in by others and check out www.huxracing.com hux has done this swap a few times.

please excuse spelling and grammer errors, im tired, as for anthing i may be incorect on, hell it was all off the top of my head so be nice when correcting.