i suppose it's theoretically possible (almost anything is, after all--with enough time & money). but remember that the capsule expects to see a certain impedence, voltage, etc. if the ntk
is designed for different values of the above mentioned, then you'll find yourself basically rebuilding the guys of the ntk
in the process. add to that the fact that a part of the way mics w/ the m7 sound is the head grille design. granted, that's not as big of the sonic signature as the capsule, but it's a factor.
another issue would be acquiring a donor m7. to my knowledge, gefell doesn't sell them, they only replace them in gefell mics. and if you're getting a vintage one from a donor mic, i'd say your money would be better spent fixing the donor mic.
that all said, if you're going to go through with it, i think a call to any or all of the following: david royer, klaus heine, tracy korby, and bill bradley.