I didn't have time to post a link last night to Vacuum Coatings Ltd / Csientific Mirrors. They do list camera prism restoration. I'm going to get a new front surface mirror for a sceond SLR shortly - a pre WWII Ensign Speed Roll film reflex and if I get time may get them to refurbish the mirror and prism from an old Exa II.

You'd need to check whether the'll strip a prism before re-silvering, they will strip & clean mirrors although I had them make a new one for my Ruby Reflex. Their prices are very reasonable and they are extremely helpful.