Yes you can get mirrors and presumably pentaprsims re-silvered. Vacuum Coatings Ltd in London are good, I've recently had a new front surface mirror made for a pre WWII quater plate SLR, relatively inxepensive at 16 inc postage.

You'd need to remove the old coating, this is usually done with acid which doesn't affect the glass itself, then after cleaning the item is re-silvered, usually these days it's an aluminium layer but just as good if not better than silver. The two surfaces entry/exit would need to be masked off.

Once you got it back you'd need to paint the coated srfaces of prism here I've not thought what would be ideal yet.