First, if you wanted to go fast(er), you should have gotten the VG30E motor instead, but that's not really helpful.

I'm not really sure what 89-90 240sx parts will match up to your hardbody motor. Your first problem is that your motor isn't carbuerated. It has throttle body injection, if i remember right. That carb-looking thing is actually your mass air flow sensor. That arrangment kinda rules out getting an intake. By the way, the "E" in KA24E stands for injection. You could try bolting on an aftermarket header for an 89-90. It should bolt on to the motor, but I don't know about the steering column clearance, or if it will bolt up to the stock exhaust. An underdrive pulley might free up some HP, as well as a bigger exhaust.