Where are the smaller transformers?
They might be mains filters but it's more likely they're pulse transformers used to driver either the MOSFETs or as a feeback for the low voltage side but an opto isolator is probably more likely for the latter.
It's probably a good idea to find out the frequency of the transformer but you still might want to rewind it because you might want a different output voltage and you might not require all the other outputs. Even if you end up rewinding it, I strongly advise keeping a record of the number of turns, wire gauge and phasing.