As far as engine balancing goes there's not a whole lot you can do about it without resorting to extremes like drilling the counterweight and fitting tungsten plugs. The mods the car guys do to their crankshafts simply throw them more out of balance and don't have any real affect on helping get the mixture into the crankcase. They sure look pretty though . The only mods I would do to a crankshaft would be to put a slight taper near the end opening and a small bellmouth simply to avoid turbulence as the mixture spills out into the crankcase. Polish the inside passage to get rid of any machining marks and build up the end underneath the carb to give a smooth transition for the near 90 degree bend the airflow has to take. Personally I'd avoid making any changes to the crank timing because there's such a wide difference between makes of similar purpose engines it doesn't seem to make any difference. It's only when you get to car/marine engines that there's much difference in the closing times compared to aircraft engines.

One thing that can really help down that end is the underside of the carb barrel. Ideally there should be a smooth transition from the barrel diameter to blend in with the opening in the crankcase. Even more ideally the transition should be made at an included angle of no more than 14 degrees. This allows the air flow to slow down without causing turbulence after reaching it's peak velocity through the venturi (carb barrel) and regain some of the pressure it lost. The higher pressure then helps it get through to the lower pressure when the crankshaft port opens. Talking about turbulence, a bell mouth entry into the carb helps here too. Any turbulence at the entry to the carb reduces airflow.

For the rest of the engine cleaning up the passages to deburr etc to give a smooth airflow though the liner ports is just commonsense but not polishing them. Changing the timing is hit or miss (and non-reversible) but necessary if you're after all out power, especially with a proper tuned pipe. One way to change the timing but not touch any plating is to machine/file a slight taper on the piston crown to lower it's edge at the port by the same amount you'd normally raise the port. The compression sealing area on the piston is usually well down (comparatively) from this mod so shouldn't affect the seal at all.

The other area where good gains can be made (and my favourite ) is playing around with compressions. Choose the fuel you want to use (mainly nitro content) then alter compression until you find the one that gives the highest revs on that fuel. With one of my engines I had a 22% increase in HP using zero nitro fuel with just a simple bit of machining. The hard part was experimenting to find the ideal .

Another thing with cylinder heads is that some people have found a very worthwhile improvement converting to tapered seat plugs similar to what some car engines use. A standard threaded head can be converted to using the Nelson taper seat plugs because they use a much larger thread size.