The most common place for a crack is between the valve seat and cylinder. An old timer told me that you see more cracks on the intake side than the exhaust side because the metal is stressed by being alternately heated by combustion and cooled by the intake charge, but I don't know if it is true. Whether it is worth repairing these blocks depends on whether or not you can find both ends of the crack, how far it goes, and how many there are of them.
This is a typical valve seat to cylinder crack that you can see with your naked eye. Another common crack is seen on the right between the bolt hole and the water passage. Many flatheads have these bolt hole to water passage cracks and they can usually be ignored unless the crack runs to both sides of the hole. Some people plug or weld these cracks and some don't.
This is a bad repair job using plugs, I think. I don't see any threads where the block broke away as would be seen with a proper plug, and in real life, there is a bronzy cast to the repair making me wonder if brazing was used. I can't imagine that brazing wouldn't have melted away, but this engine had so many bad mechanic things in it, nothing would surprise me about it. I even found lead shot in the crank case.
The same engine with another bad repair. Neither of these repairs used sleeves and they clearly did not follow the cracks to their end. To be perfectly honest, I don't know how the engine ran except that it is hard to keep a flathead down. There were three easily visible cracks in this block and since I only got it for the Merc crank, I simply haven't looked closely for more nor has it been magnafluxed. Some things you just don't need to know.