Assuming you have the head on the bench, carefully rotate the cam until it's loose (no tension on the "lifters").
Then turn the adjuster shafts until the tappet is as loose as possible.
You should be able to take out the 4 cover screws on each end and pull/pry the cover (with the frozen adjuster shaft) out of the head.
Then you can get the offending adjuster shaft in a vice and break the nut loose.
You'll at least save the cam "cover" (bearing) this way - new adjuster shafts are easy to find.
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