Note:

The speakers are 45 ohm, and you would need to deal with the room stations and those tiny resistance volume controls.

As to the door station I've changed them out to a conventional door bell, or to a different system, that ties into the phones, by putting a new speaker behind the door station grille (old speaker is most likely rotted away anyhow).

Fun stuff. Most clients decided to remove all the M&S, and patch the holes.