I think a 31cc gas engine might be overkill for the GSP Hellcat. The plane flies nicely with a .91 two stroke or a 1.20 four stroke, so a 25cc gas engine would be more appropriate. This plane has a lengthened nose, probably 1.5" - 2" longer than scale. With the OS 120 pumper, I had to move the battery pack way back to get the plane to balance, so if your engine weighs 4.5 lbs, then you will need tons of lead in the tail. I used one mini servo for the air control valve, standard servos for the ailerons and throttle, and high torque servos for the elevators, rudder, and flaps.