I have the older 80 inch H-9 Cub and fly it with a .72 Saito. Even with floats it is way plenty power, and will take off at 2/3 throttle, cruise at 1/4-1/3. I think a .56 would do a fine job, a .65 for a little more, and if you really want to be able to throttle back, the .72 will allow a scale 14 inch prop (so will the .65). anything bigger is really throwing money away. If you want to avoid cutting holes, mount it inverted, works great. If you're an OS fan, the .52 would give you scale performance, a .70 would give muy mucho performance. Any 2-stroke in the .46-.55 range would also be plenty.