Make it fast around corners. You won't be able to make any reliable HP if you don't plan on spending much money. And BTW you don't have 120whp, you don't even have 120 crank hp. If I'm not mistaken, you should be making around 110-115 at the crank, so that translates to about 99-100 whp.
If you're insistent on making your stock car faster without swapping, just buy a ZEX nitrous setup and do a 50 shot.