It's not a rubber O-ring, but a metal cylindrical bushing. Well, presuming it's what I think you're referring to, it is part #3 from the ePer parts list:
The bushing pivots over a spherical bearing that is press-fitted onto the end of the prop shaft. The bush then passes through the donut and into the flywheel. There is a rubber seal, but it doesn't function to centre the donut.
These are photos from my prop shaft.
The cylinder bushing are possibly still available (on backorder with Centerline, US, so maybe enquire).
The seal, you're right, I haven't seen available anywhere.