We have a huge amount of experience with Bang & Olufsen ribbon microphones and offer services for the BM2, BM3, BM4, BM5 and BM6 models. We have seen hundreds of B&O microphones over the past 15 years.
These microphones suffer from a few typical problems, the most notable of which is very low output due to weak magnets. We can replace the magnets in all models with strong Neodymium units. This will substantially improve the signal to noise.
A common fault with the stereo microphones is that the plastic rotating part becomes brittle and snaps. We can fit a brass replacement which is nearly unbreakable and will also improve the grounding connection between the top and lower microphones, which reduces hum. You can read a bit more about this repair at the Xaudia blog.
We also of course can replace ribbons and convert most models to standard XLR output so that obsolete cables and connectors are no longer required.
Some links to additional reading are below.