There were two types of MK 26 capsules for the M 221 B series. The earlier type used a "push-pull" transducer with two electrically active backplates. That capsule type can flatly no longer be maintained any more at all by the factory.
Whether an individual MK 26 capsule of the second (and more common) type can be repaired or not depends on exactly what is wrong with it. Unfortunately, that isn't knowable until the capsule is disassembled and inspected at the factory. Since some essential parts aren't available at all any more, it can even happen that a capsule will reveal a certain defect which cannot be repaired, which also prevents the capsule from being reassembled as a usable capsule any more at all (e.g. if certain internal parts are cracked, and they crumble when the capsule is opened). Therefore Schoeps has a form that they send out, in which they clearly state that while most microphones can be serviced or at least restored to some level of functioning, a total loss is also possible.
I personally would avoid the MK 26 capsule because sooner or later, it will almost certainly become unrepairable if it is not already. Along the way it may lose the ability to be set to any other pattern than figure-8 (the knob will turn and the mechanism will go clickety-click, but the cardioid or omni patterns aren't produced, or aren't produced correctly). Also, the frequency response of the MK 26 in the cardioid or omni setting was never as broad or smooth as the single-pattern or two-pattern capsules. However, its figure-8 setting did match the (rare, single-pattern figure-8) MK 28 capsule.
Let me just say that I am quite surprised to hear that a CMTS 501 stereo microphone (which is from a considerably more recent series) should be unable to be repaired. I have one of those myself, which was overhauled about two years ago, and as far as I'm aware, the capsules on which it is based (a specially adapted version of the CMT-series MK 6--which I also have a pair of, and which were also rebuilt at the factory just a few years ago) are all basically capable of being restored to a very high standard. That doesn't mean that every repair is necessarily practical economically (e.g. if the microphone is run over by a truck or is immersed in salt water for a long time). But for the more or less normal range of repairs and maintenance, I understand the CMTS 301/501 and the CMT 30/40/50 series overall to be on the list of models that they can normally restore to original specifications or better.
--best regards