Upon closer inspection, that looks more like a crack than a casting flaw. Disassemble engine. Clean thoroughly with degreaser, especially the crack. Might have to use brake fluid in a spray can to the blast out the crack, then shove some sort of oil resistant epoxy into the crack and that hole.follow the directions on the tube of whatever you buy, theyre really important since the epoxy will be in contact with oil products.
I patched holes in my gas tank with a specific oil resistant epoxy 4 years ago. Don't know what type, but its can be sanded and painted as well. My patches aren't leaking yet.