yes there is a way to do it. called a Helli coil. most auto parts stores carry the right size. just take your bolt with you to the store . yes it is metric. there should be some instrions in the package , should have a tap , setting tool . drill bit. ido belive. good luck .
If I recall correctly there isn't a lot of material around those holes. By the time you drill one out big enough to be tapped for a heli coil... there might not be anything left. Might be better to just drill and tap the hole the next size up and get a bigger bolt
A longer or larger bolt would be hard to find for this application.
I have seen many broken valve cover bolts due to over torquing, but never a striped out hole unless the hole was damaged during the removal of a snapped bolt.
The fact that the valve cover is basicly rubber mounted will allow SOME imperfections in a repair, but the distance from the head to where the bolt goes through the cover will multiply the effects of the hole not being squared.
Hi Goldtop, the only problem I see with the longer bolt suggestion on the valve covers is that those bolts have a shoulder on them to keep you from tightening them down too far and maybe warping the cover or otherwise damaging it. Would probably be okay in other applications where a shouldered bolt is not required.