So, there's a chunk missing out of the camshaft retainer, where the head bolt goes in. Basically, the bolt was torqued down too hard, by my big dumb gorilla hands and it snapped off deep inside. In the process of removing said bolt, it snapped off a chunk of the hole. I figure my best solution is to go get a new, either entire head from the junkyard, or possibly just the camshaft retainer. What do you think my best option would be? Also, if I went and got a new cylinder head, what should I do to rebuild it? I know it's just a D-series, but I want it run clean.

 

A pic of the damage would be great.
Trying to visualize "a chunk of the hole"
You will be lucky if u can find a Z6 head in a junkyard.