As I recall, the Royal .40 & .45 were copies of the OS .40FSR and .45FSR. I did know fliers who used Royal .45 cylinder and piston sets to replace the more expensive OS parts. They were a drop-in fit.

The actual Royal engines tended to have lower quality parts, but ran well if you got a good one.

Since the FSR series of engines, OS has gone through the SF series, the FX series and is now into the AX series. This indicates they are an old design. I would offer $20 for the engine. You may need to replace the bearings after all this time and all the rubber O-rings. Once it is de-sludged, it should run fine. You should be able to use Royal or OS FSR parts. You might be able to find them on E-Bay. I'd check RC Bearings for those.

A 10-6 prop is the correct size and I'd use 10%-15% fuel. Since it is an ABC type, don't run it in a rich 4-cycle, use short 2-cycle runs for break-in.