I believe that you have the 295 9 hp motor? Clutch work would be your best bet, leave the motor alone. I got my 350cc E-Z-GO motor up to around 16hp or so, and running 35ish. But it all depends on what you are looking for more power or more speed?
You can go to a heavier spring in your drive clutch, and if you don't already have it you can go to the 28 degree driven. The driven clutch will say 36 or 28 on the helix (the back of the clutch)