I had to do this exact same job on mine due to 2 broken studs (discovered when fitting the SRM bit, and explained the weepiness), and it has been OK for years.
I made a guide jig from half inch plate with the 4 quarter inch holes very carefully marked out and drilled in the right places (I clamped the stock for the jig to the original sump plate), attached it to the crankcase by the 'good' corners, and used the 'empty' holes as guides to get the busted bits out. Drilled out to just under 85% tapping drill size for the original thread, picked out a few remnants with a scribe, drilled to final tapping size, ran plug tap down them - and was lucky enough to get away with it. But I didn't have a buggered helicoil to contend with - just the old studs.
I found it very worthwhile to make the first drill bit into a home-made end mill or slot drill by grinding the tip off to get a flatter-ended cutter that didn't tend to wander and/or fret against the guide, and it was also helpful to have shortened it as much as possible to get 'rigid'.
In this case I'd follow the same routine, drilling undersize to start with and increasing to the size needed for the helicoil tap. Which would mean taking the jig off to enlarge the guide hole, maybe more than once, and doing the sequence several times. You should end up with a clean threaded hole that will still accept the intended helicoil if there's a god; but as he's not always at home when we need him, you might have to go bigger, and settle either for a stepped stud if you can sort one out.
My SRM gizmo came with caphead screws which I saved or some other job, because if ever there's a place where studs and nuts (nylock or double nuts even with lockwashers), is a must-have, this is it. Maybe SRM have moved on by now, as I'm sure a good few people will have mentioned they didn't like screws where studs are obviously far better.