I think when dahicori said he "set the timing" he maybe means he rotated the engine to #1 TDC in preparation for disassembly? Maybe not that he had timed the engine in the sense of configuring the static timing settings.
I didn't see this myself firsthand, but I heard anecdotally once of a D24T engine that had been seriously overheated, blew the headgasket, and then when the owner went to disassemble it they found several loose headbolts and at least one broken one. Perhaps this can happen due to the aluminum head's expansion when extremely hot that puts extra tension on the headbolts and could cause them to stretch or break.
The good news is if that is the case, the remaining part of the bolt might unscrew easily enough once you get the head removed. It didn't break due to being seized in the block and shearing as you tried to unscrew it -- it broke due to the forces on it during the overheat event. Hopefully the broken off section has a stub sticking up that you can grab with pliers and try to turn. Shoot some penetrating oil down into the threads beforehand too so that if it does start to turn, it will have a good chance of being removed fully.
