[Arran]

Vlad;
It's possible that the tar/bitumen using to pot the box may have lead to the failure of the transformer, either that or the paper insulation, one or both may have been acidic, or hydroscopic, and corroded the wire. One nice feature of the metal box is that a more modern replacement transformer, or choke could be hidden inside. I think I melted the tar out of the box upon the suggestion that it could be repaired, I even went a step further and attempted to peel the paper back to find the break, I did not get very far. Somewhere I have a plastic container full of the laminations from this transformer, which I had the idea of maybe finding a substitute transformer bobbin that would fit the iron. The choke substitute is an interesting idea, though you do loose some element of amplification, I think I worked on a Canadian Westinghouse set that had a similar arrangement to your family battery radio from Ukraine, I never thought that the Soviet equivalent of a 1N5G could be used for audio output tubes.
Regards
Arran