Thanks for the input. I didn't notice any pitting or wear on the balls or the spindle. I will get confirmation of the threading, then look around for replacement parts.
The bike spent the majority of the past 30 years stored in various garages & sheds, rather than being ridden regularly. I assume the corrosion is due to moisture condensing in the frame, then accumulating in the BB and sitting undisturbed for a long time.
Any recommendations other than a vintage replacement - e.g. sealed BB - & where to get?