re the broken stud, I would drill through an m8 nut until it passes over the stud and have it welded across the top, then work it back and forth until it binds gradualy increasing the arc, i have freed many seized bollts this way over the years, if you continue to force the issue when binding starts you will risk shearing it off completely
warming the surrounding ally up with a hot air gun wont do any harm either
If you did happen to go down this route, disconnect your CDI and generator electricals prior to welding as a precaution
apologies if you know this stuff already