Are we talking about a gasoline or diesel? Can you provide a picture so we know where it is? There are hi temp JB weld products in the auto store. But if you have tried these and they do not work, I think you are out of luck. I am going to assume this is an Atomic 4.Can you use a high temperature epoxy sealant as a temporary seal around a leaky headbolt that penetrates the water jacket? If so, what product would you recommend. Traditional thread sealants and JB weld do not stop the leaking.
And what is it leaking? Just raw water? If this bolt is somehow securing the head, it would have torque specs. If it is a true head bolt, I would be inclined to go to the root of the problem if you intend to keep this engine. That would mean removing the head and seeing what is going on inside. Possibly drilling and tapping new threads and getting a new bolt and new gaskets. A4s are very solid engines and can take a lot of abuse, but the raw water cooling system on some can be a problem. As you probably know, these are very simple and the Moyer folks are a world of help if you need parts and advice. FWIW.Engine is an Atomic 4. Head bolt is in center and is also one of two bolts that secures alternater bracket.