Doc, the US41 should fly the P-51, at 20lbs, very scale like. You won't be trying to hover the WWII fighter, so excess power would not be a necessity, just a nicety.

Since you've already got the US41, I'd say go ahead and use it. The Poulan 46cc is an excellent conversion engine, but I'n not sure the side exhaust will fit inside P-51's narrow nose even if a wrap-around Pitts muffler is used. The US41 is rear exhaust and should fit much better.