Since the job of a boiler expansion tank is to prevent over pressurization, I would replace the tank. As rusted as it looks I don't think it will hold up to very much pressure. New tank may cost $50 to $250 plus installation.

Looks like electrolysis, I recommend replacing.

Is this a job any handyman can do, or do I need a HVAC person?

A plumber or a handyman with basic plumbing skills can fix this problem.

If the leak is only coming from the valve and not the tank itself, then you could just fix the valve. Though as already mentioned, a tank with that amount of rust will have to be replaced eventually.

Twintech Heating & Cooling

https://twintechheating.ca/

A new tank is $30-$40, and it's an easy DIY project. In addition, the bladder in an expansion tank only lasts so long, so the corroded tank you have may not even be functioning properly anymore. Replace it.

If you have to ask who you need to have do the work, then it isn't a DIY task.Dealing with rusted old house plumbing is not necessarily an easy task. When all goes well, it isn't difficult, but knowing just how much persuasion is appropriate and where to apply it is a skill. The best approach is to find a patient handyman who will allow you to observe and assist

"He's got a little rust showing on the welded seam. Wire brush it and hit with some primer."

Did you see the second two pictures?

OP: Replace it!

Did not see them.

If the house is directly connected to a municipal water supply then an expansion tank is not needed. Removing the tank is another way of solving the problem.

kudzu,

Exactly correct, at least on hot water heaters.

I’m replacing the tank but it’s been tricky. The boiler is from 80s and I assume this tank is similar in age. The only relevant info on the sticker is company name (Extrol), max pressure (75 psi) and numbers (140/90) that didn’t mean much to my contractor. He did eyeball it to be ~14 gallons capacity.

Any thoughts about how to find a suitable replacement?

If it were me, I would replace that valve with a ball valve. I would think you could find a tank like that at a plumbing supply house or online. pressure tank

When you say "boiler", do you mean a hot water heater or radiant heat? Picture of the sticker?