[Arran]

I think that it is a fairly straightforward single ended, pentode based, audio output stage, so you don't really need an expensive Hammond unit. in a pinch you can use an output transformer from an AC/DC set, or even a six volt filament transformer, or a line matching audio transformer. if you need something larger. one off of a junk set with a 6F6/42/6V6/6AQ5 would also be a match. I believe that AES, or whatever they call themselves now, sells a generic audio output transformer made in Mexico, that is cheaper then the Hammond brand one.
You could also have a go at repairing the original, you may be lucky and find that one of the leads broke off internally.
Regards
Arran