If you have engineering experience you may be able to drill it out in situ, but it's not easy and I doubt you would have snapped it off in the first place if you had the skill level to do it. Unless there are any Zeddist near you comfortable with performing this delicate operation I would whip the head back off and take it to a local engineering shop.
What ever you do DO NOT drill the remaining bolt and use an 'easiout' style stud remover as 99 times out of 100 they break off and then you're facing a much bigger bill to get a man with spark erosion kit involved, and that is pretty specialist kit - even I don't have that
I really wouldn't take any short cuts getting this sorted as this is the sort of problem that can rapidly escalate into a seriously expensive shit.
Before you do any more work on your bike buy a good quality torque wrench or at least borrow one. These M6 bolts should have no more than an absolute maximum 8ft/lb , which isn't a lot.
I'd happily sort it for you but you're a little bit far from my workshop, so unless you travel around the country you best get local help
