Cobalt drills are the thing for hard materials. Be aware that they are easy to snap and need plenty of lube.
Unfortunately the quality can differ greatly, I bought some Milwaukee ones which won’t do hardened steel, unlike snap on ones I have but at double the price.
TBH, unless it’s something unusually difficult ordinary drills ought to suffice, unless it’s something like work hardened stainless
If access permits welding I would be welding a stub onto the broken portion followed by a nut rather than resorting to drills. But sometimes, there's no option.
Thanks all - ordered some Bosch ones and see how we go. I have drilled out on bench before but hoping to do this in situ provided the manifold gasket is not also leaking