So what caused the failure? That looks like a fatigue break. Did the whole chunk pull out? So I'd guess the plan is to stick or MIG it? Id get it lightly tacked where ya want it, then make it nice and warm with a propane torch. That'll make it easier to weld (warm) and also dry the moisture out. Dont forget to clean the crud out; even on the inside so the process doesnt pull the junk up into the new weld.
Wishing you the best on this one and hope those motor mount holes line up "exacto" LOL. Didja consider a torque chain to the upper control arm shaft? 'Just sayin... Bill