Can't help you but congrats on your new machine. Got pictures?
Even rusty and dirty I know I would like to see it.
I don't know if anyone has attempted to replace the MOP on a machine before. My understanding is the original process was to imbed it in the japanning while it was still wet then sand everything flat. I think replacing it could be done with care. You might get reasonable results by shaping the mother of pearl carefully to fit the hole then gluing it in with black epoxy to fill any gaps. There is also a black superglue available that may work.
Rodney