Well.......the plane recomends a 61 to a 91 in the 2 stroke catagory. My personal opinion is its a little heavy compared to a 4 stroke, and the 4 stroke is more powerfull. But that is neither here nor there since you got what you got. On the up side the 91FX will accept the same header as the 61SF so, if need be you can put a tuned pipe on it. Cool! Also up your nitro and if the current muffler has a removable baffle, toss it. You will pick up about 200 rpm. Go with a low pitched prop. OS reccomends a 13/8 or 13/9 Nooooooo!!! At least a 14/6 or I would check out a 15/4 The low pitch will help you spool up quicker with a larger diameter. The larger diameter will give you more wash over the control surfaces and more torque if you want to try some TR. I could be wrong on a 15" prop but I don't see a problem since it is a 4 pitch. Somebody else will probably step in and shed some light on that or you may just have to experiment. Good luck, Joe