cal.45acp, I think I have your answer. At least if the taper on your crank is the same as a Ryobi. If so, what you want is a prop adapter from CH Ignitions for a short shaft Ryobi. It has a unique tubular through stud that goes through the prop adapter. The aluminum prop adapter itself floats on this stud, and is held against the the crankshaft taper by the propellor and propellor nut. It's a slick system for a very short shaft engine.

See pics.

I would like to add however, that I have used the same method as Capn John on many engines and it works fine. Just use a hole saw to cut out the center of the magneto, chuck it up and dress it round, and use it behind a prop adaptor designed for engines with magnetos.

Good luck,
AV8TOR