The P 51 D that BarracudaHockey saw fly at Ocala is mine pictured above. I was amazed at the performance I get with a DA 50R, a larger engine is not necessary unless you plan to load it heavy with scale detail. If you are going with 4 blade props a big electric motor or gas motor 80cc's or bigger is required. I am flying my TF B model with a smaller Varioprop (19.9 inches Dia.-17 inch pitch) turned by a Rimfire 50cc brushless on 12s lipo packs.
Bigger motors will turn bigger props or more blades. They will add weight which means higher wing loading so the comparison between the lighter plane at 25-27 lbs. or bigger motor and heavy plane (30-35 lbs.) comes out even at top speed unless you go electric, in which case very high speeds are possible (140 mph+).
I recently flew my H 9 P 51 D at OTOW with 3 other warbirds ( a TF Mustang, a Corsair, and a P 47) all with equal or bigger motors than mine and I had no trouble keeping up with them. In fact after we landed I noticed some engine tuning going on.
Another consideration is that the engine compartment was designed for an engine that was never released and the smaller the motor you install-the easier it will be. Pilots who go for big motors will have to engineer it themselves and I can say from experience with the TF and the H 9 Mustang that the TF has more room for bigger motors and their mufflers. Also engine and especially muffler cooling needs special attention to detail in the H 9, some thing that was not as important with the TF plane. I have flown several of them for years with no extra baffles, etc., while I had to angle mount my DA 50r in the H 9 to get the muffler's manifold into the cooling airflow from the chin scoop to prevent vapor lock and dead stick landings. And a baffle was need to prevent muffler heat from reaching the carb. I ended up remounting the engine in three different positions with 3 different mufflers so that it would taxi around my yard without vapor lock. It will now run on the ground for an entire tank (32 oz's) at idle and full throttle. In the air I have no problems.