The newer .91 2-strokes are small case engines, built in a .61 crankcase. The will turn a .61 size prop, in particular a 13-6 or 12-8 faster than a .61, or will turn a normal 14-6. I have had 2 4-Star 60s with an OS .91FX and a Magnum .91XLS, both with Ultra Thrust tuned mufflers, and both flew great. With a .61, they are a nice, sport plane, but nothing great. They are fairly large plane and really handle the larger engine easily.

A .91 4-stroke would be an excellent engine. A 4-stroke .61 would not be enough power.

In any case, I recommend clipping the tips by 1 rib on each side and adding an inch to the rudder. The plane, built stock, is more like a trainer. Lose 1 rib and it rolls faster and is more fun overall. The only landing difference I have noticed is the clipped wing version lands better. The stock one tends to float and wallow around.