Whenever your hop up an engine not originally water-cooled, lighten/modifying the flywheel is mandatory. The flywheel from air-cooled engines has fins that work moving the air and conducting it over the cylinder. Of course this causes drag and uses power from the engine that, on a water-cooled motor, could be better used at the propeller. Besides, taking off the fins alleviate the engine weight and allow the engine to accelerate quickly, at the cost of a little high idle (there is no free lunch on world, even at the hobby world). In the chapter Preparing a R/C engine there are tips on how to do this and we recommended that, after prepared, the flywheel must be balanced. However, the commercially available hobby balancers are designed for propellers much more light than flywheels and are not strong enough to take the job. Jeff Thompson again sent to us pics of the one he made. Very simple and effective, as the pics show and an absolute necessity for anyone intending to prepare an engine. Lets see what JT has to say about the construction:

I made the base of the balancer out of alum and I used a long drill rod for the flywheel to ride on. I used epoxy to secure the rods to the top of the alum bracket and put a small piece of fuel line on each of the ends to keep the flywheel from rolling off the ends. The shaft that goes through the flywheel is also drill rod. And to keep the flywheel secure on the shaft I used round teflon stock and cut the one end on the lathe so that it has a taper to fit into the flywheel, just as it would on the crankshaft. The other is flat as it would be on the collet side. You want the teflon pieces to be snug when you slide them on the shaft to secure the flywheel. I made this one up years ago and it really does a great job for me.

JT