I believe Durst employs a rear silvered mirror not a front surface mirror. My Durst F60 had a similar degradation to its mirror and I went to a hobby shop and bought a thin mirror and a glass cutter and it worked out great.

Something like this. I only needed 4x4 inches. The one you have is larger I am bet

http://www.joann.com/round-glass-mirror-6-/prd53117/