For example, it this was not tightened properly...although would affect one inlet and one exhaust and that is not what the photo shows...or would it affect one before the other...?
I had an exahaust valve snap off and do much the same thing to the right hand side of mine a couple of years back. Put a new piston, barrel, head and valves on it.
It's interesting that it now sounds a lot gruntier than before, is smoother, and burns virtually no oil now so I suspect one of the exhaust valves wasn't seating correctly.