RE: giant top-flite p-51 mustang
You might also consider a 4-stroke gas engine like the Saito FG-36. It produces as much power than the Zenoah 45 and is smooth and quiet. It cost more, but runs great and is very fuel efficient.
You don't need as much weight in the nose of that plane so a heavy motor will just make you move everything back and may require the addition of weight in the tail. If you plan on a retractable tail wheel then it won't be a problem.
A DA-50 would be a good one if you want to go real fast and a smaller 35-40 cc gas if you want more scale flight.
Jeff