[Dacious]

If the amp does not sound bad, I would not be too concerned. Paint the exposed rusty areas with some rust conversion paint to hinder it's progress.

Eddy currents can show up as noise almost like radio interference. If you haven't got that rest easy. I have one transformer that had surface rust - I just brushed off the loose stuff with a soft brush and painted it. It is still OK years later. An armature or transformer winder can take them apart, clean and relacquer them if it gets bad.