Where is the crack, and how long is it? If it's a relatively short one on the outside of the block, it would be possible to drill a small hole at each end to keep the crack from spreading, V-groove it with a die grinder, and braze it. Just be sure to get the metal clean enough to accept the brass. If it's too dirty, the brass will just pile up on top instead of making a water-tight bond with the block metal. Internal cracks, like from the cylinder into the water jacket, etc. are much more difficult to repair effectively, since they're exposed to combustion heat and more expansion/contraction. Welding with nickel-alloy rod or installing a series of threaded, overlapping plugs to "sew" the crack are more reliable fixes in that case.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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