[SpudRider]

That's a common metric bolt, you shouldn't have any problem replacing it at Ace Hardware, et cetera. Please clean up the cam sprocket, and post another photograph of it. If the keyway isn't damaged, the cam sprocket is good, and you merely need to replace the bolt. I would also gently run a thread tap through the camshaft to clean up the internal threads.

The camshaft, bolt, washer, and cam sprocket are all Yamaha parts. However, if necessary, you can get a brand new camshaft from China for about $10.

Spud