The Saito .91 is a very light engine, only 20.5 ounces which is 3 ounces heavier than most .46's. I have 2 and have them on a Morris Hobbies Top Cap profile and a Goldberg Extreme 330. Both planes were slightly tail heavy with the .46 or the Saito .72 I originally had in them. The .91s fixed that and added a bunch of power.

I did have to widen the motor mount slightly and the engine sits all the way back. To give the carb air, I drilled a large hole through the profile and removed the foam for the tank in that area. The carb throat sits just about on the profile so I was concerned about breathing. On the TopCap, I added washers for some right thrust. The Extreme 330 has the right thrust built in.

I also changed to KrazyLegs, fiberglass/nylon landing gear. They are longer and don't take a set if you plop down hard like an aluminum gear can. I am using 15-4W APC props and needed the ground clearance. These are the same profile gear that are included in some OMC kits.

Drop me an e-mail and I'll send you some photos of my installation.