Well I don't think spinner is worried about the fact that it won’t fly as intended or be as graceful. He's tasted brute force power and now he's addicted. There's just no saving him. LOL.

The OS55 w/stock muffler will have plenty of power in the Ultra Stick 40 for unlimited vertical and it will "harrier in"(float) in flaps& crow mode with that engine. Even with this engine you could pull off or flutter the large control surfaces in full power level flight with it propped for speed so watch your speed especially in a dive.

I totally understand spinner's desire for more power because its just plain funny to see a plane go strait up like a freight train or instant power to yank it around. I do put the biggest engines and fuel tanks that I can cram in to my some of my 40 size planes but it comes with a cost. Count on extra strength mods, time to do it, better servos and hotter landings.

Ultra Stick 40 has short landing gear so a 13" or 12.75 diameter prop may not last long. I ended up running a 3 blade with a Saito 82 and a OS 55, I don't remember the sizes of props I used. I cut a hatch under the tail for the batteries so I didn't need to add weight to the tail (you may not need to do this if you loose the heavy junk aluminum engine mount that comes with the kit and use a glass filled plastic mount). CA coat the wing mounting dowels so you don't get wood to wood rubbing, mine started burning from the friction!

You could put an OS 91 FX in it but you would have to be extra careful with speed and landings. The fuel tank space is limited.

Note that the OS 61 is actually heavier than the 91!