Thread insert installation is easy IF you know what you're doing and are using the right equipment.
... the problem being (nothing personal OP) is the guy that just ham fisted and cross threaded that bolt PROBABLY doesn't have the necessary finesse.
I find it funny when I see people bragging about how good they are at removing broken bolts and/or installing inserts... to ME they're bragging about how talented they are at breaking bolts.
MY talent lies in getting the bolt out WITHOUT breaking it. (I used to rebuild electric motors... if you WANT to screw up a bolted connection, run current through it. I have removed more broken bolts that I will admit to).
You DO NOT want to drill this out with a hand drill... that being said; I use a hand drill most of the time... BUT this is case of "Do as I say, NOT as I do". I've had a lot of practice... DAMN now I'm doing it
... and I don't remember EVER, as in not once, removing a broken bolt with a left handed drill. Not once
I've done lots of heli's over the years, it's not a difficult operation but it's easy to f-up if you're not careful, don't understand what you're doing or look for short-cuts.
Removing stuck fasteners is another story. Pretty much every snapped bolt I've repaired was because someone else had been at it before me, or it's seized by orange locktite. Cross or wrong threading, over-torquing, and orange locktite are the most common. Removing them is a bit art, and a bit science, each circumstance will require it's own solution.
The case
@mimico_polak needs to fix sounds like the threads on all the fasteners may have been mangled by a novice, either cross-threaded or over-torqued. One fastener is snapped and stuck. The case if soft metal (aluminum) and the bolts are hardened steel, so the case will usually take damage.
From my armchair, I can see 3 solutions to restore the case after removing the stuck bolt. Carefully inspect the condition of all the tapped holes in the clutch cover case, if in doubt, treat them all.
1) Drill and tap to the next larger size (M8?), then replace flange bolts with hex head fasteners to maintain clearance on the outer case.
2) Drill, tap, and helicoil the existing holes and replace all the flange bolts.
3) Make the drive to Lindsay with your case.