hmmm how hot does the exhaust manifold get? secure it on with come of the metal epoxy weld?
It'll get
very hot... few hundred ºC at a guess, especially if the engine is working hard...I very much doubt metal epoxy weld will be up to the job.
Is there anything of the stud left sticking out, that you could turn with something ?
EG file some flats on and have a go with some molegrips (on as tight as your hands can squeeze!)
You can get some stud extractors that fit over protruding studs - was looking at some recently.
Otherwise, looks like drilling out, and removal with a reverse-thread-type stud extractor.
Or welding a plate on permanently.
I suspect room to do anything is going to be your main challenge....