"I just bought a colchest mascot 1600 (and this machine is a replica colchester) only to find a broken gear in the headstock."
Do you have particulars about the broken gear like OD, ID, Diametral Pitch, #Teeth, Material, Straight or Helical cut, etc, etc? Maybe I can make you one. Would for sure save you some $$s.
Have you considered getting it repaired?