I'm surprised none of the Rotax specialists couldn't supply rings. If you need a workshop manual for the exactly the right Rotax engine I have one on CD if I can find it, although you can get by with just about any air cooled Rotax manual.
Les Harris told me Rotax wouldn't let him have engines on credit so he had to buy them a few at a time. This also largely scuppered his plans to make the majority with electric start.
Les specified wider chain & sprockets than for other Rotax engines. Some owners fitted standard narrow Rotax 19T gearbox sprockets, but if yours matches the rear sprocket & OEM chain spec it may be one of the ones I had made!
Rear spokes are a problem due to the slightly unusual lacing pattern for the Grimeca hubs. From my experience stainless spokes are too brittle for this application even if thicker = I had so many break & so often it was a major contributing factor in my decision to sell my G80 !!!!