I've built two and a half of these vehicles, internal geared bicycle hubs don't work because the power pulses from the engine rip them appart, you could use a 3 speed hub if you had a good clutch and a fly wheel with alot of dampening in it so the power to the wheel was smooth and consistent but I have yet to see one last more then a few miles, however I've only seen a few bicycle free wheels fail in the 100's of super mileage vehicles that I've seen and again they fail because of a lack of dampening, it's like putting an impact wrench on the drive train.
Brigs and Straton engines use a splash oiler, this does not give the rings enough oil most of the time so you either need to plate the piston, rings and sleeve with something, use an oil additive that leaves a film or ad an improved oil pump that can spray the underside of the piston, spraying oil like this would also help with cooling.
Your engine shouldn't be running long enough to really over heat so shaving the fan blades off the fly wheel and re-balancing the fly wheel can help alot, it would be well worth it to go over your entire engine and balance every rotating part.
I've never done work with altering cam shafts but it might be worth looking in to having it reground.
Stock carburetors on your engine are crude to say the least, you might look in to replacing the stock carburator with one from a 75-100cc motorcycle, your engine is going to be turning alot slower then a motorcycle so you are going to have to research what size intake is ideal for your engine size and speed and match it accordingly, fuel injection would be nice but is going to cost alot and be alot of work.
Exhaust is important as well
Autolounge.net | Calculators | Exhaust Pipe Sizing with give you an idea on exhaust pipe size,