Plan A: take to engineers/ bike shop & get them to do it~ shouldn't be too expensive & then THEY are responsible if all goes pear shaped.
Plan B:
Should be able to do yourself with power drill.
A few tips~1) use quality cobalt stub drills, they'll be hard enough to tackle the bolt no sweat & as stub drills are shorter than standard, easier to hold square to workpiece by hand (& less likely to snap)
Cheap drill bits are a waste of money for drilling steel.
2) centrepunch very carefully, the closer you get that punch to the center of the bolt the more likely you are to pull the job off without damaging the cylinder heads thread.
Start with an 1/8"/3mm drill, then work up through sizes.
I think they're M6 bolts, tapping drill size for m6 is 5mm, don't go bigger than this or you'll cut into threads~ whats left of the bolt should be removable with some prying with a small screwdriver or similar.
3) if you do cut into threads a little, can usually be cleaned up good enough to go with an m6 tap.
4) If you do fluff the drilling up, you could still get it helicoiled so it's defo not scrap!
5) alternatively drill a 3-4mm hole in bolt remains & use screw extractor~ i've had mixed results with these, so prefer just drilling out.....bear in mind if you snap the screw extractor off in bolt you're in deep doggy dos as they are (or should be) hardened steel.I did manage this once
Good luck if you DIY, just take it slow & methodically.