The AR engines were not in any way supercharged. The real differences between that engine and the standard 61 were the use of crankshaft sealing rings which was supposed to help keep the rear bearing from rusting. It really didn't work.
It's difficult to recommend prop sizes due to the variables of the pipe setup, fuel, etc. Start with a 12x8 and work from there. Ideally the engine should run in the area of 10,500 rpm for best results.

Good Luck.