[Arran]

I don't understand why the speaker has to be replaced, if the audio output transformer is defective. The mechanical design of the original transformer is a bit unique, but that doesn't stop one from using a new one with the original speaker. I seem to recall that the plate load impedance of #45 tubes is similar to #42s/6F6s/6V6s that one that works with a push pull pair of those will work with the #45s. Anyhow the one you want out of that selection of Hammond transformers would be the 125D or 125E. Antique Electronic Supply has a generic audio output transformer that is less expensive, but I don't think it has the tapped secondary, you could also see if a NOS one of the Hammond, or Stancor multi tap ones shows up on fleabay. Another option is to try a line matching transformer for a P.A system, there are a lot of those around, new and used, cheap, what's more they have a big core in them so should sound good once you figure out the wiring.
Even if you end up using a new speaker don't toss the original, a new output transformer can mount on the frame of a new speaker, and so can a filter choke, if needed.
Regards
Arran