Engine head crack...

Hi I have 99 sonata with 138k mi. During fixing belt tensioner, I guess the mechanic made a mistake because I couldn't find such crack before.

Anyway, engine head has a very minor crack and near the crack, I found it's cleaner due to engine oil leaking but not much (very little). I had no problem in driving but I'd like to know whether it will be serious problem or not.

If I need to replace head, where can I get decent used head?

Thanks for your comments, in advance.