The technical term is fuel tank vent valve, but as stated by others - it’s part of the tank and not a serviceable part.

JB weld and other epoxies may work temporarily but in the long term they won’t hold up - the expansion/contraction of the tank combined with the vibration during driving will eventually cause the patch to fail.

You can try a fiberglass repair kit for gas tanks but I wouldn’t expect that to hold long term, either.

I’m sure there’s a fix (where there’s a will, there’s a way) but considering the tank would probably need to be dropped to make a proper repair, I’d just get another tank (and probably fuel pump if it’s got a lot of rust on it).