My LHS gave me a really good deal on a Magnum XL .91RFS and I'm thinking of installing this on my Big Stik .40. As per airplane manual the recommended engine for a four stroke is .60-.80 cu in and an O.S. .70 FS is highly recommended for this plane. I compared the specs between this 2 engines and only a slight difference on weight and RPM range. But the O.S. is about $60.00 more.

I would like to do basic aerobatics with this airplane with unlimited vertical. I'll appreciate it very much if you can give me any inputs.

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.