I had not noticed this thread.
Some do go metric but that becomes a pain latter on when some one else gets their hands on the bike.
Helicoils will be fine just use long ones.
For head bolts you should be using 2.5 or 3 d
The length of a heicoil is defined by the diameter of the thread so the cone in D, 1.5D , 2d , 2.5d & 3d
For head bolts you really should go 2. d or better.
They can be done by hand with a normal power drill & hand tap as you are effectively drilling out the hole 1 size larger and the drill will follow the original hole well enough and the wire is a sloppy fit on any thread profile.
However you do need to be careful when you start the tap off to make it nice & square.
IF you have an alloy head then 20 to 25 ft lbs will be fine, any more will collapse the holes around the bolt making it very hard to get off.
If the head is iron the 35 to 30 ft lbs.
If you must go metric then make sure you use metric coarse threads as std metrics are a bit fine for anchient alloy so you are looking at 1.25 or 1.5mm thread pitch.
And if you are going metric you may as well jjust drill & tap for the next size bolt rather than drilling a even bigger hole for the helicoil