Never before heard valve knocking started yesterday...

The valve seat came loose? Did it beat the piston to death or did you shut it down when you heard the racket?

There are cylinder head repair shops all around my part of Alabama so I'd think you could find something close by. If it were mine I would go by the oldest machine shop you can find and explain your problem. You might save $$$ taking the head off and have them replace the seats, probably old bronze now, and grind your valve edges. Install new valve seals too. Put the head back on yourself. That way your Z stays in your garage.

Edited October 20, 20231 yr by siteunseen