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.