My Hobbit's driven pulley was pretty gnarley inside. The ramps were all chewed up and it was starting to stick open. So, I decided to do something with one of them GY6 driven pulleys.
I didn't really want to put a clutchless variator on my crank and do all the complicated stuff to the gearbox to get the GY6 clutch working, so instead I machined a little spacer thing with the correct taper to mate with the drive shaft taper on the gearbox. The spacer then mates up with the innards of the GY6 driven pulley once you have removed all the bearings. Welded the parts together, and it's on!
P.S. no that crusty shit gearbox is not on my hobbit; just a spare :) (edited)