Loads of vids on youtube, my first option would be cut a slot in the end with a dremel, try a bit of heat and screwdriver it out. Next would be slip a nut over it as snug as poss and weld it on, then put a socket or spanner on it. Drilling re-tapping not my fav but is an option, depends on your skill level really.
I did the same to a cbr 600rr a fue years ago. If you can get some heat into it with some WD40 and if your lucky mole grips will get it out that's what I used. If not weld a nut on is the safest bet. I dont like the idea of cutting a slot into it due to the fact it will be nearly impossible to get a good hold on the screwdriver between the radiator and the exhaust port too much risk involved with that for me ie rounding the thing off even more.
Had a same problem tyke when I changed my headers over and snapped one when doing back up! Mole grips and lots of heat does the trick
If it's steel in aluminium then use 50/50 acetone/ATF. Orientate so it can soak, keep applying for a few days if necessary. If still tight use neat acetone. A glass eye dropper from the chemist is a good applicator. Heat helps, if you dare use, depends on the tolerance.
