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Look for a transformer with the correct primary topology (push pull or single ended depending on how many audio output tubes there are), Primary impedance (should roughly match plate resistance in output tube data sheet), secondary impedance (should match speaker voice coil impedance which is usually the same as voice coil resistance), and should be rated for the max audio power the radio is rated for (or the tubes are rated for if you can't find an audio power spec).
Note: not all radios regardless of speaker type (PM or field) mounted the audio output transformer on the speaker, some mounted it on the chassis instead. If the speaker was subbed and they were smart enough to wire the field coil correctly then they probably made sure an output transformer exists within the set.
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