[383hq]

he proper way to do it is to remove the head. Usually the most expensive up front costs are the cheapest long term costs..
Particularly given the "concern about restoring the M6 with inserts is how close it might come to the combustion chamber and related weakening of the head metal between the injector hole, valve seats and thread repair insert"

But to answer your question, yes standard (hardened) drill (slip) bushes exist..

however If you must proceed in-situ drill and tap/thread repair, and you have access to a lathe, take a short standard fastener (M6 ? M8 ?) from the hole to be repaired, and drill a fastener thru hole, say 1/8" or 3mm. insert this as a drill guide to get a decent centre. (broken bolts tend to not be smooth and the centre of the drill wanders.) Use rod if stripped of correct diameter

For M6, 1/8 or 3mm is a small diameter, and may only get 70mm from a long series drill. Use a low grade fastener (Grade 4) for the guide -don't use a 8.8-12.9 Socket head cap screw...

I'd be marking the depth on the drill too, Hate to make it a through hole...

if available use a LH flute drill and hammer function, you may get the drill itself loosen/remove the broken fastener, without needing the screw extractors (may save a step), but will likely depend on whether broken bolt has bottomed out.

Oh, and Use a very sticky grease, the swarf / chips produced may kill the engine if not 100% removed, (or alternatively bottom the new fastener resulting in the soft aluminium thread stripping out)

also worth noting that M8 at 25Nm is likely well over 1000Kg of clamp force depending on fastener and lubrication, and could be as high as 1900 Kg f...