A little extra power is good, more is better, and too much is just about right!

You may want to mock it up and see if that .61 doesn't stick out the nose too far. Beef up the firewall so you don't lose it. Better to be a bit nose heavy, but you can place the receiver battery to help with CG. Take offs may take a little longer to rotate and landings may require keeping a little more speed so you don't run out of elevator. Sounds like fun!
Try it and let us know.
I've got the H-9 P-51 Miss America with a Saito 100. Its good, but I'm thinking 25cc gasser might be about right.