It is very easy to replace the glass, I have done it twice, the first time the mirror was replace with a 2mm glass and it worked great, but it cracked after a few months, so now I have put in 3mm glass and glued the little plastic thingy you see in the pictures. The only problem with using the 3mm is that you can't switch to night view using the little button at the bottom of the mirror.
Go to a glass shop with your old glass and they will make it for you.
If need more instruction, I'm glad to help.
I bought an 8 x 10 in. sheet of 2 mm glass at framedestination.com (for a few bucks) since the auto glass shop didn't have that size. They said thin glass tends to break during cutting but if I wanted to chance it they'd give it a shot. Since I had enough for several attempts, I bought a cheap handheld glass cutter, traced the shape from my reassembled broken glass with black marker onto the new sheet. I cut it close enough on the first attempt, was able to cut the larger curves and carefully crush off the corners. I was prepared to take the dremel sanding wheel to it, but the unfinished edge is unseen behind the front of the frame. I applied super glue (cyanoacrylic) all around the rough edge to keep it from flaking and make it less sharp. It looks good and the night view lever works as it should.
FYI. I mounted a Series 2 rear-view mirror on the Series 1 attachment plate/knob temporarily while I've been restoring my original Series 1 lighted mirror. The problem is that the window rake angle is different and the Series two mirror looks "up" quite a bit higher and cannot be adjusted down enough to see straight "back". I've even done some "artful" grinding on the lower part of the mirror base and it improved but still cannot dial it in perfectly. I have no idea if the glass fits. I'll take a gander next time I piddle with it (a Winter project).