After making certain the engine is properly tuned, the easiest way to develop more horsepower is purchasing an oversized crank pulley kit such as the following:
Twos R Us - Parts for Toyotas from Club MR2. You will need to make a small modification to the vacuum lines. Then I would consider a different cat back exhaust. The OEM muffler is large and heavy.
Instead of body kits, improve the handling of the car and its brakes.