Discuss.
Suitable "drops" are available via E-bay for about $30. See my previous for sources for Rotax prop extensions; the Saber ones are the nicest and made in the US, the Airtrikes are the cheapest and come to you from the Ukraine.
If anyone is aware of an online "brokerage" for small run machining, let me know; there is one local machine shop I'm certain could make this adapter, but they're a little spendy (high priced Bay Area).
You could do something similar starting with this adapter from Performance 670, Product Description but it's relieved on the back for the stock flywheel fixing bolt, which cuts down the available frictional area. I'm also not certain it's big enough for the Rotax bolt circle; I have made inquiries.
And Kohler/Briggs fans could presumably do something similar.
I'm still trying to work out some way to make the flywheel to prop happen with ONE piece, a custom spool extension that used the flywheel fixing bolt and the flywheel puller holes for clamping force, but I haven't worked out all the details yet. The fabrication would be more challenging.