First choice weld it with nickle rod. Second choice is to weld it with nickle rod. You can try epoxies and may work for while. If just a seep or getting white smoke, drain your radiator, rinse your cooling system with water to get out all antifreeze. Use the old egg glass stuff, sometimes called liquid glass. Probably called something else in auto store, but it will be clear and probably have glass in name. Can buy it cheapest at pottery supply, think they use it as some sort of mold release or something, but easiest just to stop at AutoZone or wherever and pay the $6 for little bottle. It will be about most expensive radiator seal on shelf. Dont know why as its pretty cheap chemical. Add the jar of this to radiator and fill up with water. Run until leak stops and no white vapor from exhaust, making sure you dont run the cooling system dry in the process. Can leave it like this during summer if you want, but when you need antifreeze again, drain the system and rinse before adding antifreeze. This stuff reacts with antifreeze, but what has already hardened to fill crack wont react.
I've seen old tractors go years and years using egg glass and my neighbor, Chuck the drunk, drove some little GM car with quad four engine for months after using this stuff, then sold it to a relative. I imagine he forgot to mention that he had used liquid glass in radiator. Hope they drained it before adding antifreeze for winter.