I bought a Global Hobby Fokker D-VII that had the motor in place already.
Ebay member Monstergogo sells the EMD motor, but not sure he has any parts, I asked about a bolt-on prop adapter and he sent me a link to a collet.
The adapter is availavble as a machined part to fit the motor into an FMS plane, but it's way too long, would need a threading die run down it, and has a hex portion that would have to be smoothed off.
That's my fallback, 15 bucks. For a bit more they will customize it.
Don't tear yours apart on my account, I'm just trying to line up the parts before I need 'em.