ORIGINAL: dadkins
Ya the prop that is on there looks like about all you can fit. I would try and get that old motor running again - vintage = cool.
Royal's were not known to be powerhouses. A TT Pro will outpower the Royal by a good margin. Royal was at one time made by Thunder Tiger, and later by Leo. It is a clone of an OS FSR, which is a little down on power when compared to an OS SF/TT Pro. They dont have very good bearings inside, so if it were to be made to run again, you'd want to replace the bearings. If that Royal is ABC and still has decent compression when warm, then I'd replace the bearings and run it for now. If its ringed by chance, a new ring could be had from Frank Bowman for about $11.
OR if you want an engine that will run forever (literally), you could trade in that Royal engine on a brand new Fox engine for 1/2 off the retail price. The big-case Fox .45/.46 engines are quite strong and worth the money. They may be less desirable looking, but they make up for it in power. Plus they're made in the USA!