i would say that a 91 would be a good engine for the 40 size p-51. but i think the optimum fourstroke engine choice would be the YS63S, Saito 82 or the OS70 Ultimate. i've seen the plane with the first and the last and they have been rockets. for two strokes, the 46AX and 50SX is a great engine. TT Pro 46 is also good. but i know less and less about 2 strokes as i pretty much buy 4 strokes now.

using the heavier engines would also require you to really strengthen the standard retracts, which are known to be on the soft side. while it's not that much trouble to bend them back, it does get tedious if you have to do it every time. i'd stay light, my friends plane with the TT Pro 46 rarely needs to be bent back even if his landings aren't as pretty as mine (both on paves runways). plus, keeping it light and not overpowered will help the overall durability of the model.