helicoil wont do any good with the bolt still in the head.
Ive had marginal success with left twist drill bits. With any drilling on a snapped steel bolt in an aluminum base the biggest problem is getting a clean center hole drilled into a usually non-flat surface without slipping off and damaging the threads thereby making it even harder to get the pieces out.
get a set of left twist bits, start with the very smallest and be careful to drill in the center. Then step up each size of bit. Along the way one of the bits may catch enough to remove the broken bit, bit if it doesnt, eventually you will be left with no broken bit. THEN, maybe you will need to helicoil it.