Sounds like was seized pretty good maybe you can try on of those bolt extractors that goes in your drill? Keep soaking it in PB Blaster while you work on the carbs and come back to it later.
If you don't get it out, maybe you'll get lucky and find that the idle was set about right when it broke.
89 VN 750A - Given to son-in-law
79 CB 750K-sold 3 May 21
78 CB 750K
77 CB 750K
77 GL 1000 x 2
77 CB 550F
Holton, KS, US