ORIGINAL: khodges
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.
Thanks, I've been looking at the Evolution also. Do you know if a pitts muffler is available for it? I was also looking at the RCS 140 with the rear exhaust port as a possiblility. The G20 looks like it would mostly fit except that the carb would probably be sticking out the cowl. It is suppose to be equivalent to a 120. I want this cub to fly scale and if possible not hack up the cowl, so as you suggested the evolution might be a good choice. What little I've read about the evolution 26 here on RCU hasn't been very positive. I've also have seen very little mentioned about the RCS 140 as far as reliability and performance.
Dan