You could try a washer and nut on the back side of the broken hole. Use a screw long enough for the washer and nut but no longer. I do not believe that would interfere with any moving parts back there. If that works you can take your time finding a replacement headstock casting for a permanent fix.
Mark V 520 S/N 09-11-01 completely upgraded to Mark 7, with all SPT's & more.
Model 10ER S/N R64000 first one I restored on bench w/ Shopsmith metal ends & retractable casters. Has Speed Changer, Model 4E Jointer, Jig Saw with lamp, a complete set of original accessories & much more.
Model 10E S/N 1077 oldest one I have restored. On bench w/ Shopsmith metal ends & retractable casters.