For whatever it's worth... The crack you describe is EXTREMELY common. In my own case, I had my 59 open about 10 years ago - to do a valve job & some other maintenance. Sure enough, mine also has a crack that runs from the #6 intake port, across the deck, and down the cylinder wall (a little). I remember that it concerned me at the time. I have since quit thinking about it, as it has never caused any problems. My engine does not overheat, never needs water/coolant, runs year-in and year-out without having to add water/coolant, doesn't smoke, doesn't use oil, and doesn't have any other bad habits that I know of. In my case, I'm thinking 'what's to fix?'