Long or Short shaft Ryobi ?
The long shaft is easier to convert.
The hub is not threaded. It has a clearance hole for 3/8"
The original 3/8" nut holds the prop, hub, and flywheel.
By theory, it is possible to use a shorter hub with a long shaft engine than a short shaft one. On the short one, your hub has to be long enough to have 5/16" holding threads for studs front and rear.
Enjoy,
Jim