Do you HAVE the plane yet? If so, you know how light it is. Most people are using 91 four strokes in the H9 P-51 and I plan on using a Super Tiger 90 in mine.
I have two Moki 210 engines, one WAS in a H9 Cap and the other is in a Byron Pitts at 15 pounds.
If you want to put the Moki in a P-51, get a World GS.