It's a temporary fix, but whole Black Peppercorns will work. They swell up. Just dump in the radiator while the engine is running......you may have to add a lot of water until the pepper gets to the cracks, then it will swell and seal it up.
Otherwise, pull the engine, strip the block, grind, pre-heat and weld! The trick is in the cool down.........it has to cool as long as possible.
Use a puddle/spray torch to apply a powdered nickle alloy or just weld with S/S.
