Re: striped bolts
The boltholes in many motors are not tapped to the bottom of the hole. You only have a short bit of the thread in the hole, makes it easy to pull the studs right out. All of the mounting, intake and exhaust, and head stud holes should be tapped. Harbor Freight has a pretty nice set of metric taps for about $12.
Actually, using a torque wrench is good for all the mounting nuts, too. When a 6mm only needs 8 ft lbs over reefing on the nuts can cause the studs to break as well as the chance of pulling the threads out.