Girard-perregaux 1960s Automatic

Bought this watch at an auction..Pretty cheap and runs, but certainly needs service.However, the metal O ring in the back which screws and holds the case is pretty damaged and tends to fall out.I don't know if this can be fixed or if I have to find a new one.And I don't even know if that's possible with such an old watch.I'll take it to my watchmaker, but any suggestions would be helpful.I looked on ebay and was not able to find something compatible. By the way, a definite redial, as I am sure you have all noted, but I think this redial was probably done decades ago..you can tell by the lettering.It's not crisp like you see with a truly original dial.But I think the rest of the watch is probably authentic
Thanks Artis