Nice plane, a .91 size 4-stroke will do fine for scale flying with a some extra for emergencies. I'm helping a former student right now with that plane putting in an old OS.91fs. I have three H9 fighters (F6F, P-40, P-51) and all fly with .91 4-strokes. Its said the perfect match is the saito 1.00. People have put up to 20cc gassers in some of the H9 fighters. I personally wouldnt do that due to the extra weight. The stock landing gear works mediocre up to about 9lbs total weight. More than that, and they suffer. The struts are spindly and the plane sometimes does a chicken hope down the runway. Looks embarrassing. The saito 1.00 will help keep the weight down. I modified mine by putting the servos all up by the CG, made a new servo tray. I didnt put the servos in the tail like the instructions stated. For every oz in the tail, you need about 4 oz up front to account for it. Except for my P-51, that was a swap meet plane. It will still fly alright with tail servos, I just dont like them back there.
Edwin
I missed the 4-stroke gasser point completely. Jester has it right.