Is the crack horizonal near the bottom of the water jacket? If so, it is likely a freeze crack and will not get any worse. I would just clean it well, vee it out alittle with a grinder and repair it with JB Weld. Texture it while soft with a piece of 40 grit sandpaper pressed into the epoxy and it will look OK.
That should hold it fine until you find another engine, or it will probably be leakfree for the rest of it's life, maybe yours too.
