Restoring a poster print using watercolour
I don't have any pictures, but I once saw a "restored" print technique which amazed me.   The damaged painting was valuable, so the owner didn't want to modify it directly.  So he bought two appropriate sized frames and from their parts assembled a single frame with two sheets of protective glass.

Then with the painting behind a single sheet of glass, he painted his repairs on the glass.  I suppose that the medium was very forgiving because any mistakes could be wiped away before they dry and reattempted as often as needed to get them right.  Eventually, he had the look that he wanted.  He then waited for all the paint to thoroughly dry.

At which point, he took the painting and glass out of the frame.  Added the second sheet of glass in front of the painted one and reassembled his artwork.
In this way, the repairs on glass are themselves behind glass and the finished work is mostly indistinguishable from the original undamaged work.