I do not believe that a .91 swinging a 13/6 @ 10 grand has anymore thrust than a .65 would on the same prop at the same rpm. The differance on a 13/6 could only be due to the ability of the .91 to turn that 13/6 FASTER, or the .91's ability to turn a bigger prop ( which would be good on a slow airframe) at the same rpm. I vote for a slower, higher torque engine turning a large diameter prop, over some high rpm engine that takes 3 prior owners, and a couple gallons of fuel to get to run right in this 72" bipe. 4-strokes come to mind here, but are not your choice, nor would be mine either. There are plenty of really good 2-strokes that will do what you want, and the LA-.65 might be one of them. The OS .91 FX would probably last you the rest of yer life, and fly the wings off of that bipe.