A .91 works great. I have had them with an OS .91FX and a Magnum .91XLS with a Performance Specialties Ultra Thrust muffler. Can you spell vertical. Knife edges all day.

I put the rudder and elevator servos under the stab, side mounted the engine, full cowl, clipped 2 ribs and added back rounded tips, moved the gear to the wings, added length to round off the fin and stab. It is now a mock WW II Kawasaki Tony.

Excellent flying plane.