[Arran]

Speaking of breaks in a coil that may very well be the problem with the field coil, sometimes the wire breaks near the end for some reason, or the solder joint breaks down, it's worth investigating and trying to repair. In my experience it's very seldom that a field coil will burn up unless the output filter capacitor develops a dead short, I had one radio where this had happened. In any event unless the speaker is a weird size or shape you should be able to find another if the original can't be repaired.
Regards
Arran