Ever since my son got into RC cars I've been reading the engine section and there seems to be one magic ingredient for hopping up their engines which always works. Imagination! They'll slice a couple of grooves through the crank counterweight (!), polish the crankshaft then say they've gained another 10,000 revs .

Personally I don't think there's a great deal that can be done to get any worthwhile improvements without radically changing the way the engine is supposed to be run. The only reversible mod (and the easiest to begin with) is to optimise the compression for whatever fuel you want to use.

Woops, I just had a thought, there's one mod that can help. The carbies are very inefficient below the barrel (venturi). Ideally they should taper back out to match the opening in the casting as far as possible at the crankshaft inlet port. The better they do this then the closer they come to returning the pressure drop back to atmospheric (or at least whatever pressure is above the barrel). The top of the carb should have a smooth bellmouth with radiused edges to keep down turbulence and get the greatest airflow into the carb. Then below the barrel it should expand at no more than a 14 degree included angle to avoid flow seperation and turbulence with a final cross section area as large as possible. This is where it tries to regain the original pressure. Most carbs or venturis are about 60% efficient (below the venturi) but a very well designed one can be up around 90% efficient. Higher pressure means it'll flow more easily into the crankshaft port and more air means more power.

Blending in the end of the hole through the crankshaft below the carb with epoxy to give a smooth radius for the incoming air to make a nearly right angle turn will also help.