Chain is fine. The main part of the gear is fine on. It’s just that thin outer gear which appears to be pressed onto the the main one that is damaged.

Me too Chappie......no ideas on that one at all

Something caused it to break, it didn't just fail for no reason.  If you turn the motor over by hand & watch the chain & gear, you'll see (maybe) why that gear is made that way. When someone screws up the cam timing & then just hits the start button, things break. I had one in the shop after the local Yamaha mechanics (LOL) worked on it . Removed the cover, turned the motor by hand & watched the cam drive the chain/gear, instead of the gear/chain driving the cam. Had another after the 'mechanics' fucked up the crank  by turning the motor backwards to line up the timing marks, jackasses never admitted it either. Sold him a motor with a good crank & took his for parts.