If the threads are just a little wonky and not completely destroyed, you could try chasing them with an M6 tap.

But, that kit should work if chasing doesn't. Make sure the hole is deep enough for the tap to fully cut threads deep enough for the insert, though. The 'bullet' type taps usually have a ~1/4" (~6mm) taper at the tip, so the threads will not be cut the full depth of the hole. Also be careful that the tap does not bottom out in the hole. You will also need a t-handle (not included in the kit) to hold the tap along with some cutting oil (3-in-1 works well). Both can likely be had at your local hardware store, including the Helicoil kit. Go slow, follow the directions, and you will be fine.

Use blue loctite on the insert when installing, and switch to a stainless bolt (no loctite on bolt) to prevent dissimilar metal corrosion. Helicoils are not as strong, and not as deep as the original threads, so don't torque on it too much.

If the Helicoils fail, you can always step up to solid bushing inserts like BigSert from TimeSert, but this is a pretty low-torque application so the coil should be fine.