If you have a push to ground switch, you could make it work with a relay. You wire the relay so the push to ground completes the primary side and opens the normally closed secondary side of the relay.
Les, I'm a little electrically challenged. Your description, I think is for a push to open button like the late Commando button. On P11's we have a two wire button that is push to close and points and coil. Any idea how this gets wired? My thought was to wire it as I described using a relay, but it wasn't wired that way originally. With battery and coils, it seems that any grounding of the coils will blow the fuse.
I have period Lucas wiring diagrams, but they don't show a kill button and I don't think Norton (or Matchless) ever published a wiring diagram for P11.
Any ideas?