If as you say, it is in the head, then get another head and you are OK. To my mind you still have the original engine. If it was in the cylinder wall of the block, you could sleeve that cylinder. Elsewhere in the block is not so critical, and I would think you could fix it by "Stitching" or weld it with muggywelding rod, or even use JB weld.
When I took my head to have it done 25 years ago, the shop said it had a crack after magnifluxing it. I had them go ahead and rebuild the head anyway, as I could not see a crack and it was working OK. Still working OK.
Kurt
