Ok, you said that the idiot cut a diamond inlet into the stock, and the plastic diamond fell out, but you didn't say much of anything else about the condition of the stock or rifle.
Assuming that the stock has no other big defects (deep scratches, cracks, breaks, chips), and assuming that the rifle is in 95% or better condition, then I guess I would find a small piece of straight grained ebony, find a friend who is a cabinet maker or wood worker, install an ebony diamond into the hole, and then resand and refinish the entire stock, to make the stock look beautiful. You really have no choice, since original replacement stocks for pre-64s are darned hard to find and expensive, and trying to get a Boyds or other aftermarket and starting from scratch is very expensive as well.
With the stock refinished, then you still have a white elephant, but a very nice looking one, and you may be amazed what someone will pay for it at a large gunshow. Walk around with it at a show, let people handle it, comment upon it, heft it. I'm betting you could get $500 or a little more from a hunter (who loves Winchesters, but not a dealer).
Regards,
Mannyrock