Mando, If you want speed, go with the 2 stroke. If you want power go with a four stroke. I've flow a 40 size trainer with a O.S. 52 four stroke, and a 12 1/4x 3 .34 APC sport prop. It pulls like a tractor, but it isn't real fast. I've got a Sig 4* 60 with a Magnum 91 four stroke, and again it pulls like a tractor. It has more than enough power for any aerobatics you want. Straight up as far as you want it to go, and as slow as you want. I sure wouldn't replace a 46 witha 91fs though, it will way over power the plane. One other thing to watch is prop size. If the plane is setup for a 10" prop, you may have trouble with a bigger four stroke as they like big props. I crank a 14-6 with the 91 on my 4* and am tempted to go to a 15-4. Just some things to consider.

Don