Can you describe the structure and materials of the mirror? It’s not clear from the photo. What are the yellowish-brown blotches?
I don’t see any reason to add anything. The design is quite clear. Doing something so that it does not continue to deteriorate is more important, but even that is difficult to do without making it worse. Displaying on a slanted board might be the best idea.
Where are you? Is there anyone nearby that can look at it for you?