Need more information.
The older ones are part of the radiator.
Newer mostly are plugs that can become replaced like.
Edit
Looking at you picture. Replace radiator imo
Those columns are usually pretty heavy gauge steel so I'd see if it could be saved - Drain, then remove the blank and wire brush on a drill, clean it all up, remove all blistered paint and see what the tapping threads are like. If they're ok then grease the threads and then some rust converter, then a few coats of etch primer and then white enamel/hammerite.
May return after a few years though, depends on the prep.
If the budget's available then can always replace.
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