I'm not sure about the exact displacement in CC's but I think it will be 36 or 37. That should be more than adequate to fly Cubs & Cessna's in the 80 to 100 inch range. Try a 20-8 prop to start and do some performance tests. Try to prop the engine so it will turn around 7000 -7500 rpm. That should give you plenty of thrust. Someone has likely already done one of these conversions, and can give more accurate info. Good luck.