If you can use a digital relay (essentially, a computer) to replace the mechanical relay in a pipe organ, you should be able to replace an older electronic control with a newer. The black box approach has been used (some Siemens cars in Sacto had experimental chopper controls for a time before the CAF cars were done with them), at least for reverse engineering things. One might ask (if Muni didn't provide it) if Boeing can supply documentation allowing repair or replacement with something newer.

Most of us old-audio types know about the lifespan of parts. Interestingly, the semiconductors themselves don't wear out like tubes do, and in storage not at all, but the capacitors now ... and something like the Boeings probably has lots of them. Also contacts, variable resistors, etc. - anything that moves, rubs, touches, or is exposed to crud. The cost & hassle of finding and fixing it all could easily drive somebody to replacing with a K, or maybe something off a junked Breda.