I've re soldered mine successfully. I replaced the return line barb with a AN#6 fitting. You'll need a blow torch to do it and some plumbers solder which has a higher tin content so it's a bit harder.
You'll need remove the tank, drain it and leave it with all vents open for a couple of days. Then flush it with soapy water and rinse it out several times. I connected the vaccume cleaner to the filler neck and let it run for 20 minutes or so before firing up the blow torch. I left the vaccume cleaner connected and ran it again before lighting the blow torch. The idea being to replace the air in the tank regularly so no flamables can build up in there.
The other option after the tank has been thoroughly flushed is to fill it up with MIG shielding gas. Cap off the larger outlets with a bit of masking tape to keep it in there. It's heavier than air so will tend to sit in the bottom of the tank.
Soldering the tank is pretty easy since most factory tanks are
zinc plated and the solder flows easily once you've cleaned the area with a bit of acid paste flux. Most likely you will just need to heat up the area enough to let the original solder flow back into place.
Cheers