David,
What rotten luck for you..
From what I was able to judge from the pictures you have in your album it looks to be the location/alignment tongue between the bedways that has separated from the headstock casting.
I fear you may need some professional help to replace the tongue with a piece let into and secured to the underside of the headstock. I imagine there will be alignment screws bearing on the tongue through, most probably, the rear shear so that the headstock can be slewed slightly and correctly aimed along the bed, using a test bar in the spindle socket to check progress.
All this will need to be done cold I think. With the greatest regard for a skilled welder, any concentrated heat in the area will introduce stress and potentially add to the damage.
Before any salvage work can be done you need to remove the headstock to assess the full extent of the cracking but before you proceed with that, are you quite positive that this is not some local feature masquerading as a crack in bad lighting? .It does seem to have an unlikely form to my eye.
Regards Brian
Edit. Looking again at picture 2 I can see a gap between the headstock and the bed which is of similar width to the crack and what seems to be a clamp strip below the tongue. It is almost as if the headstock and bed have been forced apart and torn off the tongue in the process.
Edited By Brian Wood on 22/06/2020 15:14:48