when using the 2 nut method as described above also try screwing the bolt in then out and soaking with penetrating fluid
if you have no luck and before you shear the bolt off flush you could try my method below it sounds like a bodge but has worked in the past.
the biggest problem with drilling out a bolt/stud is keeping things straight and drilling down the middle so heres my bodge.
get 1 m6 sleeve nut or stud connector nut or what ever you want to call it, basically its a long nut approx 15-20mm long,
now find a brake cable adjuster from a bicycle these are m6 thread with a 3mm hole down the middle
now buy a GOOD quality 3mm drill bit .
now you are ready to attack the stud
screw the nut onto the broken stud, screw the brake adjuster into the stud, now drill take it easy and apply cutting oil/lube
with any luck you will now have a 3mm hole down the middle of the stud
you can now use larger drill bits until you get close to 4.8mm ( the thread tapping size for M6)
if you get really luck you maybe able to pull/pick the old stud out of the engine case with out damaging the thread in the case
hope this makes sense