Those are the usual cracks that almost all of the old series VW/Audi/Volvo diesels have.
The general rule is that as long as you can't fit your fingernail into it, it is OK. That one looks pretty normal, though you should check it.
Are there any other cracks in other locations, like from a valve seat to a prechamber? Other cracks would be a problem.
And, is the head surface flat or warped? I think that would be another major thing to check after the history on your engine.
