I would enlarge the exhaust port and then polish it, raise the top of the exhaust port 2 mm, open and smooth the intake and transfer ports and raise the transfer ports 1 mm. In lieu of raising the transfer ports 1 mm, install 2 base gaskets and raise the exhaust port 1 mm. Do not polish the intake port. For maximum performance, polish the rod and have the crank assembly jig alignment/balance holes filled with cork or other light weight fuel proof material. Remove the oil injection and install sealed main bearings in the engine cases. Install a 1 to 2 mm larger venturi carburetor. Install a mid to high rpm expansion chamber.
These modifications will raise the peak HP and increase maximum RPM. It will also decrease low end torque.
Run 87 octane non ethanol gasoline mixed 20 to 24:1 with a premium synthetic, vegetable or hybrid synthetic/vegetable oil. Run a B9ES or B9EIX spark plug. You may advance spark timing a few degrees, but advancing the spark timing will reduce low end throttle response while increasing high end HP and torque.
Remember: you can only go as fast as you can afford.
