I have had several of the .40 size WM P-51's and built some for friends of mine. I installed 61fx's in a coupl eof them and a G90 in another. My current one is the Miss America scheme. I have a Saito 100 on this one with a 12/12apc prop. Runs real nice. I added some weight on the inside back area of the airscoop.

Just bef up the firewall inside and out and run some extrat epoxy in the retract mountin blocks. I do this regardless of engine size. Should be fine and flys very well.