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are you able to get the manifold off now ? is there any bit of it sticking out or is it broken off flush ?
which manifold and which bolt towards the rear? Front ?
ok once the manifold is off you can check the depth of that hole using the other exhaust hole threaded bolt holes in the head so you do not drill too deep and pick up a set of new exhaust bolts and center punch the broken bolt and then drill it out and tap in new threads with a tap and die
if you can not get a drill in there good use a 90degree drill