The only part on an aluminum radiator that can be repaired by Tig welding would be the tanks. The heat necessary to weld will destroy the fins or tubes A hot melt Glue gun might surprize you though, I've seen it done
Consider pinching the offending tube at the header plate to reduce or eliminate flow in the single tube and apply a little hot melt glue. It sounds hokey but it works. Used a lot by the circle track crowd in a jamb.