Assuming you mean the MC012, just get a new one and send the old one to me for disposal (the capsule contains a toxic gas that can be environmentally harmful.)
If you're sure it's the FET, that's an easy replacement if you can get the part. I believe it is a 2SK170 if my memory serves me, which Digikey or others should have. It comes in several different versions depending on the current rating. I'd suggest buying two and replacing the one in the other mic also to keep them as similar as possible.
If this problem is the FET, I would replace it in both mics with matched 2SK170 pair, as Crazydoc has suggested. If you are in US get a matched pair from Kevin Carter of K&K audio just for a few cents more. The whole surgery should not take more than 15 minutes.