You also need to open the drain valve to do a proper flush.
The liner is plenty hard and won't have a pilot hole through it, so no worries about damaging it IMO. The only real problem will be whether the special tap will cut the threads deep enough before hitting the liner. If necessary, you can grind down the end of the tap to make it more nearly a "bottoming" tap, but I don't think it will be necessary. Most Helicoil taps are already a "plug" configuration, rather than "taper".
Most likely, you won't need to drill at all; the "skinned out" diameter is a very close match for the "tap drill" diameter and the tap will ream out the difference as it cuts the new threads. But be sure to hold the tap square to the hole while getting it started.
I agree with Art, wouldn't even try JB Weld or similar. Too much heat/vibration/pressure; plus too much to lose if it lets go.