Your only problem is going to be prop clearance. A Mag .91 will be a great power plant, but the landing gear is too short. It barely clears a 13 inch prop. You might want to get an after market gear or one from a Big Stick 60.

You also may need to mount the elevator and/or rudder servos in the rear fuselage for balance.

Finally, you'll need to switch to a larger, .91 size Great Planes engine mount.