[Arran]

 I think that I would do one of three things: The first would be to get a new cone and recone the original speaker since 10'' cones are usually easier to find, salvaging a 60+ year old speaker cone often does not end well so I would only do that as a last resort. The second thing I would try is see if the field coils could be swapped between speakers, unless it is welded in place the pole piece can be pressed out of the rear bracket, the coils removed, exchanged, and the pole piece pressed back in. The third option may be to add a smaller choke or resistor in series with the field coil of the second speaker and use it that way.
Regards
Arran