[Arran]

Open audio interstage transformers is a common failure in early AC sets with push pull audio output stages, such that I'm planning on testing out a solid state substitute I fount in the AWA Old Timer's Bulletin, whilst not authentic it is a more economical way to get a radio working that doesn't really have a high monetary value. The field coil is a little more of a problem since it's part of the power supply in these sets, in some sets it's in the B- return line so it's not as critical as a filter choke. They can be rewound, in whole, or partially, it takes time but it's a bulk wound coil, as long as it's close enough in D.C resistance it will work. There are ways to make a S.S sub for the field coil as well, or to use a iron choke, but since you would still need a replacement speaker anyhow there isn't much to be saved there.
In most sets I have the speaker field coil is usually fine, it's the primary of the audio output transformer that fails, though not as often as an audio interstage transformer, my model 96 had an old Hammond replacement installed and it has failed as well.
Regards
Arran