How Much Damage could possibly occur..............

If it failed at the right time the heads and all upper stuff would be fine.If it broke as that plug fired it would send the piston down before any contact.the block can be welded if its just a whole but it would have to be in the right spot.I know a guy that had a gun hole welded up in his block with no problems.If the piston got up into the head and bent the valves then there could be some head damage.It's always worth a shot lokking inside.As they say "prepare for the worst but pray for the best".