Dean, you're right. Ideally you want to swing a large prop at a slower speed because it's more efficent than a small prop spinning fast. Why? Because, in simple terms, the fast prop beats the air and the slow one slices through it. Generally an electric power system that has been optimised for a plane this size would swing a 15" or even a 16" prop. The problem here is the classic pattern airframes, particularly ones with tricycle retracts don't lend themselves so well to large diameter props. So, if you can make a 12x12 work that's going to help. Another thing is to try a geared inrunner in a clamshell mount bolted right to the original glow mount. It should fit with ease. Also, if you can, look to using 6S instead of 5S. Higher voltage and lower current runs cooler. A little later, I'll see if I can come up with an inrunner and an outrunner power system designed around a 12x12 prop and a 6S battery and post it here.