I don't wish to ruffle Johns feathers, but welding an almost 30 year old steel tank that has worn down thin enough in one spot to leak is like sticking your finger in a leaking dam; eventually the metal will fail somewhere else under pressure, even if you recoat the interior. This is just my humble opinion.
Unfortunately you probably won't find a direct replacement that is the exact size, but you may get lucky. Measure the circumference of the tank and then measure the amount of slack the tank straps will give you. This will give you a plus and minus size. IE your tank maybe 14" around, but your straps will allow 15"- 13" of circumference before bottoming out or not be able to do up.
I replaced mine with two five gallon aluminum air tanks. I wanted more air capacity to reduce the amount the compressor cycled; yes the compressor runs for longer I guess (45 seconds or so to refill), but it will only turn on once every hour or so depending on terrain.
I did have to cut off and reattach the tanks hold downs as the drains were in the wrong position but that was all. It also gave me a chance to replace all lines as they were suspect as well.
See post 34 below.