If you found where the gasket was leaking, it is probably safe to assume that crack had been there a while and wasn't causing a problem. There is going to be coolant under that area so it is going to be critical that the new gasket seal perfectly on both cylinder sleeves. My first suggestion is to show that picture to the people at an engine rebuilding shop. For a little extra peace of mind, I would try to wash the area with carburetor cleaner and a stiff wire brush the size of a tooth brush to get all of the carbon out of the crack, then fill it with a small amount of JB Weld. There won't be much pressure there. You're just trying to contain the coolant from possibly sneaking out. You might even ask about RTV sealer. That will stay more rubbery. I like the Chrysler gray stuff. It gets a little harder than their black stuff and will bond and seal through a little oil residue. Some gasket sealers won't stick at all if there's a little carbon or oil on any surface.
If the people at the engine shop say to "give it up", and find a different block, listen to them first. That is the type of thing they specialize in. A good welder might be able to fix the crack too. Welding on aluminum is a common repair but it should be left to the experts, especially in a situation like yours where you don't want to have to take it apart again.