remember once you break the bolt extractor you have a problem as they are hard like a tap, and like a tap the only way then is to bust them up with a bunch. Never use them myself as they tend to make matters worse.
bolt removal takes practice, or good tools, with Allen screws you have a problem as they are hard to drill, heat is usually a good start then move on to heat and CRC and finally drill them, the heat softens and loosens the bolt. Then you can often work em out with a punch or grind a bigger key into taper and drive it in.
BEST/CHEAPEST solution especially engine casings.....go to automotive machine shop and go 'here fix this'.