Jim,
For the smaller shaft sizes I've found a tapered reamer more useful than a stepped reamer. I've had mine so long I have no idea of the current prices, but that also suggests that the investment will pay itself off in long use. The one I use can fit into a 1/8" prop hub hole; the Cox half-A engines used a 5-40 thread stud or bolt, which is 0.125" OD - 1/8". The AP 09 stud may be a bit larger... A tapered reamer can also help fit props to larger shaft diameters, prop extensions, extension spinner nuts, etc.
I usually ream in from the engine side until the prop JUST clears. Keeps it centered...