I wonder if there is enough metal around the stripped hole to allow drilling and tapping it oversize for a helicoil. A lot of the threadless stems I'm familiar with don't have a lot of "meat" around the bolt holes.

If the bolt isn't threaded into a "blind" hole but the stripped hole is drilled all the way through you could drill out the hole to a clearance size for the bolt (usually 5 mm so a 7/32" drill bit will work) and then use a longer bolt and nut.