There is a few things you can do. First would be intake and exhaust. The choices are very limited though. I suggest construction your own intake kit. It will cost you a lot less, and perform just as well, if not better than any of the aftermarket intake kits for the n/a. When it comes to the exhaust, the only real hp maker would be a custom setup, but it's loud and annoying, and expensive. I suggest the Hotshot Header Kit, and the GReddy SP exhaust.
Next I would work on ignition componants and aftermarket clutch. The n/a's ignition components are shitty, and only there to do the job. Jacobs Electronic's ICE PAK ignition system is the way to go, it was dyno tested to gain 15hp on a stock 91 n/a, pretty damn good gains. You will prolly need an aftermarket clutch at this point, the factory clutch can't hold worth a damn. I suggest the ACT street/heavy duty setup.
Next, you can advance timing up to 13 degrees, it will give you some more low end grunt, and a little less high end, but if you have the exhaust and ignition setup that I suggested, you won't see anything but a gain accross the entire power band.
When you really get some money, you can finally do head/intake/throttle body work on the engine. I also suggest at this point Total Seal piston rings, and Gen IV Supra valve springs.
After the engine is built, nitrous is the answer. Zex makes a wet kit that is very simple to install and lets you choose between 50, 65, and 75 shot.
GReddy SP exhaust $500 new $300 used
Hotshot Headers $450 new $375 used
Homemade Intake $100 - 50
Weapon R Intake $150
Injen Intake $150
ACT Clutch $280
Jacobs ICE PAK $400
Zex Nitrous Kit $450
Lowest Price on Performance Parts for the MR2
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