I have a spare US B200s transformer if anyone needs it (wants to pay for shipping). Although if you were going to replace the transformer, go with a toroidal they fit perfectly (bit/top of bolt poking out) and why I have a spare coz we put in a 240v.
I found the cause, if you read the warning, with the switch in mono mode you connected both sides of the B200s together and fried it
(very much the same as if you plugged a speaker lead from A out into B out except there were also 4 speakers in the mix/circuit). Are the speakers still ok
Hopefully there was some protection in the Marshall cab switching thingy... otherwise I suspect they are toast (check them with a meter not a guit amp output signal or maybe more frying...)
When mine fried many years ago it took out only one side and the cause was a shorted speaker
Anyway fix is the same, as Sparker advises, you need to replace the output transistors (IIRC they weren't garden variety ? the dealer/importer here had parts), we also had to replace a few other bits, it's a long time ago so I have sketchy memory of what else. We also put a bunch of fuses in various places to make sure it didn't happen again.
Also, I may have said this before, there were at least 2 B200s models I know of, the one that blew was my first one and the first model here (Australia), the B200s I have now was a later, slightly improved model, I don't know what the differences are (MJMP ??) or if/how much the parts schematics are different.