If you've got an antique radiator with a stuck spud, here's one way to replace the whole bushing so you can install a new spud.
Almost all of the internet recommendations for this are to cut the spud into sections and pry them out. I think that is too likely to damage the threads in the bushing, and on the first radiator in this video, it didn't even work. Even after cutting the spud into eight sections, they were all still fused to the threads in the bushing.
I've used the scrape/penetrating oil/crescent wrench/mallet technique from this video on multiple radiators that were all 100 years old, and it worked quickly and easily every time.
You can probably use a cheater bar instead of a mallet. I haven't found one I trust with the amount of force necessary here.