If you want the best engine to fit inside that narrow cowl of a 1/4 Cub, few petrol engines can beat the Ryobi 31cc conversion. The rear carb, rear exhaust engine makes for a very long and narrow engine - perfect for Cubs.

Otherwise, the Zenoah 26cc would be a fine choice. Cubs have plenty of wing area, so you don't need to be too weight conscious with the engine.