a little hole in cast iron radiator

In my experience welding cast iron is only for experts, and not something I would do at a job site.

You might try plugging the hole with a screwed plug. The size of the hole will determine the size of the plug needed.

For this repair, you will need a suitable drill and tap. For sealing the threads, I recommend wicking grade Loctite # 290 threadlocker.

In fact, Loctite recommends this sealer for sealing porous castings, and may seal your leak on it's own.

Do some research on Loctite 290 for sealing porous castings. This will tell you what's involved.

Good luck.

That's the size you see. In my experience a corrosion hole is the tip of the iceberg. Around that hole it is most likely thinned out from corrosion. The recommendation above is a good one, but again no guarantees.

As has been said cast iron welding is a serious specialty requiring special filler rod and typically an oven pre heat of the metal.

Some people have had success with JB Weld. Make sure the area is very clean before trying any repair.