There are a couple of gas tank sealers on the market, but I haven't had much success with them. And sooner or later someone will post how wonderful JB Weld is. Of course JB Weld is gradually attacked by the ethanol in most current gasolines, so it's at best a temporary repair.

Soldering is the best, permanent fix, provided tank isn't rusted through. You need good metal to tin properly and make a good bond. However, I won't use a torch to solder. I think it's too dangerous unless you get tank almost surgically clean, and then fill with dry ice to create a nitrogen blanket.

Fortunately I have picked up 300 and 550 watt soldering irons over the years, and they have enough heat to do the job, without an open flame. They are too expensive new ($200 to $300) but maybe you can find one second hand, or borrow one from someone in the metal roofing business.

On the rare occasion I've needed a tank repair I've turned to the local radiator shop... they laughed and said, "look, a little overflow tank".

Boiled the tank out, soldered it, and painted it with radiator black paint. Modest charge and never failed again.

Follow JAL's advice on this one.
This is a case where the repaired item will be better than buying a new one.