I've waited a couple of days in case anyone else had a better suggestion, so here's mine.
I don't know of any source of panels for something of that age. I presume you've contacted McIntosh themselves just in case. I would be very surprised if they kept spares that long, but you never know.
Failing that, you could get a piece of glass cut to the right size (it may need to be toughened, the glass supplier will know if you show them the original) Then find a screen-printer that can copy the legending and fonts and print the faceplate to match the original. It'll be expensive, as setting up a screen takes a lot of work, but then if you need 1000 faceplates, each one will be cheap! (that's a weak joke, by the way)
There may be more modern ways of achieving the result of a screen-print, but whether they'll look the same I can't say.
Best of Luck with the quest, interesting project.
S.