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I suggest clamping the stud in some soft jaws in a vise & seeing if you can get the nuts loose from those studs that came out. Then double nut them & reinstall the studs using locktite & screwing them in as far as you can.

Then when you are assembling the exhaust connections, use some hi-temp anti seize on all nut threads so the next time things will separate at the nut/stud interface.

Better yet of course is to install a new stud. But either way, use antiseize!