[Arran]

An easy way to replace those is to harvest the driver out of an old headset, the impedance is usually close enough. You have to be careful with some 1920s horns, sometimes they are worth more as original, but non functional, then working but having a transistor radio speaker in the base. With few exceptions they were not paired with any radio in particular, other then the big makes like Atwater Kent. They are not the sort of speaker that one would use for much other then demonstration purposes. There are also a lot of horns around that no longer have the base, or are matched to the wrong base, I think I would be more inclined to convert one of those then to take taking an original but stubborn pot metal one apart.
Regards
Arran