The exhaust manifold is loose. Some idiot stuck an ugly, rust-prone allen bolt in the exhaust manifold. It is stuck in the head. I have sprayed PB Blaster on it, but it still hasn't budged. I assume it is 6mm, but I had to drive a 1/4" into it after the 6 slipped. When I try it, I can see the bolt flex, but not move. Not good! I'll never be able to remove the manifold or tighten it or anything unless I get this fixed, but if I break that bolt in the head, I will have a real headache.

Well, Dad gave it a try and made it worse. He slipped the 1/4 allen wrench. Now I'll have to try again with something bigger I guess. I can't even get a pipe wrench to work. For now I'm just going to put the other bolt back in and ride while I still can!

What he said ^
Really, don't lay into it with the pipe wrench just yet. If it snaps before it lets go, chances are better than even it will snap level with the hole and you'll have nothing to grip. Then it gets more difficult. Use JoG's penetrating fluid. Give it time. there's no hurry. If it comes to it, you could cut the bolt head off with a grinder, remove the header, heat the area around the bolt with a little propane torch and work at it with the small pipe wrench or big vise grips. Could do it with the bolt head still attached and the header on but there might be more room to move with it off. Take it easy. It'll come out. You'll be fine. New bolt, use coppaslip or something putting it back.