Valve guide boss cracked

You may to get some bright light and and a dental type mirror and inspect it as close as possible, maybe even probe it out with a pick to see how far the crack runs. Usually, these stop right in the thick part of the guide on each side But being I don't see any of it on the roof of the port is a pretty good sign that it hasn't migrated towards water yet. As long as it's not in the roof it is probable it hasn't made it any farther through the guide. I would go ahead and cut the guide down, I really think you'll be able to cut that one out before it gets into the damage point.