With an engine that still has the governor in place, it's not going turn any more rpm than the governor will allow. You could try a 12 tooth clutch or a 40 tooth sprocket ( taller gearing ) but if the engine doesn't have enough torque, you might lose rpm and actually slow down. I know that there's ways you can adjust where the rod connects to the governor arm to get a couple of hundred more revs out of it, but I'm not the one to ask about that. Every engine I own has the governor removed, billet rod and flywheel, and the rest of the goodies. You are at that famous old threshold " how fast do you want to go? How fat is your wallet?