On the scooter motor I had the sprocket was simply held on with a nut.
The scooter motor I've got also has the sprocket held on with a nut. Tried to get it off before with no luck, but will have another crack at it soon. I'm pleased to hear that yours works and self-rights with a 4.2:1 ratio though, as one of the Aussie builders uses the same motor for his axe and I'm sure I'd seen him saying that it needed a minimum of a 7:1 ratio. But hey, if it works! Already using 8mm chain and a 9T motor sprocket so I can keep the existing setup in my robot and have it running on a 5:1 ratio.