Just as a contribution to this conversation, I had the same problem with the speedo stopping after hitting a bad pothole in a national speed limit area: so it was not a fun experience. I just bought the rotor from a local Yamaha dealer for £22 and it should be ok now as apparently the part was known for failing on this model and so the bakelite has been reinforced with a plastic collar for the lugs. I found this post very useful for being sure I got the right part;
https://www.tapatalk.com/groups/fazerowners/speedo-sensor-sorted-t11571.html#p300468