A .56 will fly it fine, if what you want is a Sunday putt-around. If you want to do things a Cub was never meant to do, or wanted to turn a scale diameter prop, then a .65 or .72 is also a good match. I have one of these on floats with a .72, and knowing how easily it comes off the water with that kind of power, it would also fly scale with floats using the .56. Sorry I can't help much on cowl cutouts, I mounted my engine inverted. Since it is flown over water exclusively, I put an on-board glow driver as a redundant system for extra reliability, but found I didn't need it. Engine never misses a lick, so I just use the glow for starting. Here's some pics of my installation: