From, my experience with MTH repairs, and I have handled multiple of the same engines for the decade I have been doing this. Has it really been that long already? If there is a major zinc pest issue it is in a run. Typically that model has many engine that experience it. As examples: there was the chassis for an early RK that the drivers crumbled. Also some of the Bantam that had chassis swell and lock up wheels. Not all of them, but many in that run. Additionally, this engine is from late 90s. If a major contamination, it would have shown up by now, fully. With many other GG-1 owners complaining too.
The original post was more about a poor casting being painted as is and that moved into the Zinc rot discussion. I believe this is more of a contaminated surface before painting issue, or possibly some localized impurities in the casting at the surface. Unseen at the time, but has now shown up.
I am certainly not proposing a repaint, but if the defect bothers you, do not purchase. If you bought new and it has surfaced I understand the disappointment, but it is 20+ years old. G