I have a clutch actuator customer out in California who is a dealer for these engines...he also builds bikes for resale.
The other day we were on the phone talking about quality issues and he mentioned the same problem you are having.
He uses the race plates from a torrington thrust bearing to space the sprocket back out to where it should be so that the chain doesn't rub on the engine case.
If you go to McMaster Carr
and key in thrust bearing you will see what I'm talking about. It doesn't look like they sell just the plates.