Grinding, " diesel " noise under valve covers

No. Factory endplay, not sure for THAT engine, but most is in the 0.002-0.010" region.
To cause your problem, I would expect that to have increased to 0.030" or probably larger. That larger size is more easily seen with the naked eye.
With the engine off, try using a pry bar behind the dampener to lever the crank forward. Then, try levering it back. Might have to rig something up [ block of wood? ] against the frame to lever it rearwards. A helper would be handy, one watches the crank movement, one does the muscle work.