I'd get some brass pin stock that I could stick in there fairly well, and silver solder (meaning silver solder, not tin with a little bit of silver in it) them in.
That said, I'd be pretty leery of that casting. Those are big weird holes, and it may be that the voids continue on inside somewhere.
Use plenty of flux, though, and your silver solder should theoretically run in and pretty well fill out any voids... theoretically.