Yeah, I know what the book says. But having had 3 Hob Bipes I can tell you that you're going to need a 91 4-stroke. I use the OS 91 4-stroke and it's like the motor was made for that plane. A perfect combination. The Hog Bipe begs for a 4-stroke, but if you're gonna put a 2 stroke in it I would suggest an OS 61.

Hope this helps