I would have thought MIG-welding it would damage it further, and TIG-welding the crack won't solve it either. You need two new tubes.
You ought to send the photo to the frame builder and ask them for a free repair or a new frame. When you have a crack along the weld, it's obviously a fault in the weld. When it goes across like that, so that it is a crack in common between two separate main tubes, it must indicate a severe weakening caused by too much heat.
Although Reynolds 853 recovers from welding better than most steel, it is still weakened by welding, and more welding would weaken it further. The head tube in particular is quite a thick tube and it shouldn't crack like that unless there was either a huge impact or a bad error in building that frame.
A good frame builder ought to be embarrassed by that and should offer to see you right, especially if you're the original owner.