It is very difficult to weld on a frame, when the box is sitting on it. The heat from welding is many times hotter than what it takes to damage the box etc.
You want someone that is experienced in repairing TT frames in this manner. Fixing the frame is pretty easy. Doing it without damaging other things is not.
I have seen such repairs done.... Jbarca did his. But he is exceptional, and puts a great deal of thought and effort into such endeavours. Probably not cost effective to pay someone to do what it would take.