That's a malleable iron casting. Rather than TIG, have you welder grit blast, then either silver braze or just use brass rod. He should know to preheat the whole gear before brazing. Mine had two cracks when I got it and it came out fine, then I found a 'parts' winder and replaced the gear. Fun machines to run......
OH...... Do not oil the top of the square vertical bar that the needle fits into. If that gets oiled things don't go well, stitch length is all over. Did you get the manual? There is a good on online. Eli might have a copy for sale. Ewe does have copies....
God bless,
Mike