I went to my son's stash of modeling clay for a couple of blobs of Sculpey. (I use this for lots of cosmetic repairs on china and ceramics.)
There was a rope-like piece of plaster missing from around the edge of the frame so I took the blue Sculpey and pressed it into an existing part of the rope making a mold. (It doesn't matter what color Sculpey you use. I just used contrasting colors for the purpose of this demonstration.)
Then I baked it to make it hard. Sculpey bakes at 200 degrees, 15 min per 1/4" thickness. This tiny piece (and the others that follow) took about 20 min.
When it was cooled, I glued it into place with Tacky Glue.
I used to be a photo-retoucher back in the days before PhotoShop, so I'm pretty good with color matching....
At the same time, I replaced a larger piece missing from the scroll-work at the top of the mirror. Here, you can see the brown base color before I added the top layer of gold paint. (This piece had to be molded by hand because the corresponding scroll on the opposite side would have molded backwards...I learned this through trial and error! My dyslexic mind couldn't figure that out until I tried to make the mold!)
After the brown base color had dried, I lightly brushed some gold tone paint over it to blend in with the rest.
Here you can clearly see the patch but when I got the mirror hung, it was virtually invisible. You couldn't find it unless you were looking for it.
This is the first time I've tried this and it really was a simple fix. I am pleased with the results. The only thing I would do differently is to blend the patches a little better.
Good luck with your own attempt! It really was easy with Sculpey!