That small hole is meant to be there Richard, after you put the bearing in and then screw in the retaining plate a small split pin goes through that hole to stop the plate unscrewing.
This has probably been caused by the bearing being belted in crooked, I would vee out the crack and weld it and then do as Muskrat says and machine the outer and then shrink a sleeve over the top. If you just weld it I think it would crack again when the bearing was fitted.
As Bill says finding another hub would be the easiest but this one can be repaired effectively with a bit of work.