Evolution makes a nice gas engine that would fit well in the Cub cowl, the 26GT. It will turn an 18 inch prop (scale diameter for 1/4 scale), weighs about 32 oz, makes 3.8 hp, and costs about $400. The carb sits in front of the cylinder, and a Pitts-type muffler would get the exhaust behind it, so the cowl would stay holes-free. Anything around 23-26 cc will have plenty of power to fly the plane, a G20ei or G-23, or G-26 Zenoah would also be good engines, but require a hole for the carb. The G20ei (electronic ignition) is on the small end, displacement-wise, but it makes as good, or maybe better power, than the G23, which has a magneto. G23's aren't available unless you can find a used one, it was upgraded to the -26.