The G-20 Would WELL overpower the 60 size P-51! Any Maker! I had one in a Hangar 9 Corsair swinging a 15x7 3 blade and it was to fast for scale flight! The other problem you might run into is the side carb of the G-20. It would butcher any cowl. I had to cout the cowl of the Corsair and it is big and round! Your best bet would be one of the new rear carb 20cc gas engines that are out there. The only problem that I had with that was that I could not find one that would balance in a 60sized mustang with out a lot of extra tail weight. I have a Top Flite 60 ARF with a OS 110a in it. That engine weights 21ozs and it required no tail weight to balance. When you start adding 12 or more ozs to the nose with a gas then the weight needed to the tail become important!