[Arran]

The size of a filter choke is really not that fussy, but a 10 Henry filter choke would be more then sufficient, in fact it removes AC ripple several times better then a 500 Ohm resistor. The field coil performed a double duty as both a filter choke and an electromagnet, so they used more turns of wire then they would have for a stand alone choke to do the same job. If you want to go cheap you can also built a solid state substitute, a back issue of the AWA Old Timers Bulletin had an article on this, although the iron core choke is probably the simplest solution, at least until you find another speaker.
Regards
Arran