I looked into this a lot and there are a few problems. Main one is finding rear sprockets large enough for ratios greater than 3:1 or so. They're out there, but they're expensive and usually made of cast iron (heavy).
You can find aluminum Harley sprockets, but they're for a larger pitch, so still not big enough to get a good ratio.
I think Ripperton ran a belt drive, but it was custom made and a pretty low ratio for racing (IIRC).
I'm guessing Zero is making their own (or has a custom supplier). Would be nice if they sold them.