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It can be readily fixed with solder. The filler necks were soldered on at the factory. A radiator shop will do it for you or you can do it yourself if you have a propane torch and some plumbing solder.
If that is your surge/pressure tank then yes you need to resolve that issue. Generally one rust hole is just the canary in the coal mine and the tank likely has other rust issues
if this is your overflow tank then it is not under pressure and a hole at the neck presents no real problems other than you know the hole is there
Soldering a mild steel tank to a brass filler neck with lead solder is next to useless, lead solder is not going to hold to mild steel. You need to braze the filler neck, similar process, but using higher heat and a brazing rod.
Yeah, this is the surge tank. I have a good radiator shop down the street, I’ll just take it to them. Thanks.
... and if it's the original European filler neck, have them replace it with a North American filler neck.
How do I know which one I have?
The rad shop will know. If there’s any doubt, replace it. European filler necks are a bit deeper than domestic necks. As a result, domestic rad caps won’t hold their rated pressure.