The .46 performance is "sufficient". Trying to fit a .61 into the plane will likely leave you with a lot of extra cutting/fitting, but would certainly improve performance. The main goal would be to be able to swing a larger prop to provide for the movement of more air around the large cowling. I would suggest trying a .72 or .82 four stroke (eg. Saito) and a 13X6 or so prop if you want to step up performance.