Unsure of the size, but the 80's CR moto-X bikes used same type of fixing and may share common dimensions in this size.
(Attachment Link)
Listed for CRs 250 450 480 etc. Possibly a route to research.
Do you happen to know the diameter of the output shaft? It would need to be 25mm, it's mainly the rear I am struggling to find in 520 as I could machine a 530 front down to 520 pitch
Unfortunately I don't, raced a red rocket CR 250 years ago and remember thinking how neat that sprocket fixing was, and matching the road bikes in concept. Nothing I've got to measure currently (thought I had an old front sprocket laying around) so no immediate help to you.
Someone in historic moto-X must have information though.
Very nice bike by the way.
