Have you tried playing with the gearing? How often do you max it out in top? If not that often, lower the gearing for more midrange punch
Engine wise, easiest, and probably best bang per £ is a set of hi-comp pistons. After that, the 944 kit Keefy mentions works well. Allow for some carb setting work. Dyno would be the best route.
As with everything, your budget will dictate what you get. You can go to 100bhp if your wallet will stand it