You could take out that H23 and put in an H22

I don't have an H23, but I'd assume that the normal i/h/e combo would get some HP. Start with the basics (new intake, header, and exhuast) if you can't think of anything special to do.

If you plan on racing at all, look into wheel+tire combos. The entire performance of your car lays on the tires (literally!). Lighter wheels and stickier tires will help alot.