The .91 4stroke would be more than adequate. Mine flies at around 6700 feet elevation with Thunder Tiger .91 4stroke. I rarely use more than 1/2 throttle. It has 3 inch wheels on a Dubro composite landing gear mounted in front of the wing on the fuselage so there's a little more nose weight. It balanced fine with servos, battery, and receiver located near the rear of the wing bay and propeller close to where the plans would have it. Either engine sounds fine to me but of course different.