Well you sure can get into tough choices with four-strokes. For some reason 2-strokes tend to jump a lot in weight when going from .40 to .60 size and it makes the balancing equation trickier. If you're willing to do the homework, it seems like 40 size planes would be the best to stuff a .91 into. It would be a good question for the engine forum. I have personally witnessed a big success using a Saito 80 on a 40 size Ugly Stick (according to Horizon, the 91 weighs 20 grams less than the 80.)