How to repair or replace a power transformer?

Sorry to hear that.

You will need to get AC voltages from the other amplifier. I would run the transformer with no load and test its voltages, then hook it up and measure them again so you can give these figures to the transformer vendor you choose.

It may help to provide both transformers to the manufacturer. That way they will have a transformer they can test and one to take apart if need be.

This will likely cost you about $500 by the time you are done, just for the part. A pair of these amps are up on ebay FWIW...

I blew 5 power trannies on my Sophia 845's. Tech could find nothing wrong with the units. I got sick of paying $400usd for one trannie so I subbed in separate Hammond high tension and 5 volt filament trannies(put in the same one in the original) and have not had the same problem since. Best thing is that the 4 Hammond trannies only cost me $250cad which is about $160usd these days. The hammonds were far heavier as well. 

Sometimes a power transformer can have design issues- such as a spot in the part where a winding makes a bend that causes the insulation to fail. That can lead to windings shorting, the transformer thus running hotter so more windings short and so on.

Sounds like you made the right move!