[Arran]

I would see if you can get the 39-45 restored, it's actually one of the better sets for the 1939 model year. As for using the field coil speaker for something else it can be done I suppose but It would not call it practical. The first thing you would need is a power supply to operate the field coil, of around several hundred volts, then you would need to match the impedance of the speaker voice coil using a transformer, however I don't know if you could find a transformer that would match the Philco voice coil to a more modern amp. Chances are that the one in the radio chassis may have failed anyhow, so whether you replaced it or got another for a different amp you are still buying a replacement.
Regards
Arran