I've got a Zenoah G23 on CH Electronics ignition on mine. The plane balances perfectly with 3 servos under the stab. I've got several weedie conversion engines that would fit too, but I simply picked the smallest displacement gas engine I have, since the Spacewalker would be way overpowered on any of them. You'll need a side carb and side exhaust engine, as the cowl is not very long. Ryobi is out of the question. I've got several Kioritz/Echo 25.4cc engines that are also perfect fit for this plane. Homie 25 or 30cc could possibly work, but are a bit porky. You'd need to use an electronic ignition on any conversion engine. This plane likes about a 30 Oz engine with servos up front and about 40 Oz with servos at the tail. Anything with stock magneto will be easily over 3lbs and too heavy.