There is a product used in the boat repair business called "Marine-Tex". It's one of the best sealers I've used, and before you scrap that block, try doing a thorough cleanup right down to bare metal (and then clean some more!) A drill motor with a wire wheel brush, or a cup brush on an angle grinder works for me. Spread on a layer of the Marine Tex about 1/4" thick and overlap the crack about an inch on each side. As it's curing, press a piece of 40 grit sandpaper onto the patch to mimic the surface roughness of the cast iron. The patch will almost disappear. The cooling system pressure for most stovebolt engines is usually low enough that a surface patch won't leak as long as the surface is prepped properly.
Jerry
