I'm looking for suggestions about which gas engine to put on this airplane. I've tried a Super Tigre 2000 and a Moki 1.5 without good results. It needs to go on without any major modifications to the airplane or engine. I'm considering either a First Place 40cc or 52cc.

How about a Fox 2.4

I have a FPE 2.4 on mine, and it has more power than you can possibly use. I only have about a gallon of fuel through it, so it's still set very rich, but I can do any verticle maneuver without exceeding 1/2 throttle. I did have to put 6 inch wheels on it give enough clearance for a 20 X 10 prop. I just received the tall TNT landing gear, so I am hoping that gives enough clearance for 22 X 8 prop.