The only thing I can think of is drill it out and use a heli coil. I really have not had much success with 'easy outs' or similar methods to get those things out. You can try those first, but be prepared to use the helicoil.
I haven't been here so much, because my gearbox was messed up, so i had to ship it back & get another one.
The day I got the gearbox, I went for a 15 mile ride away from home since I hadn't ridden in almost 3 weeks. On the ride home, the stud didn't break.... but the "female" part of the mount - where the stud fits into - broke!! I don't have a Sad Time engine, so it might sound a little weird to you. Anyway, I have no choice but to buy an entirely new engine & gearbox for $120, since the part of the block that broke is gonna cost me $50+ itself. And I'm tired of shipping parts there, waiting, & waiting some more.
I had definitely tightened the bolts & had never had problems within 20 miles worth of riding, but with this new engine... I'm honestly going to be taking my own advice - replacing all bolts with Grade 8 bolts, replacing all gaskets, & putting blue Loc-Tite EVERYWHERE!! That prolly won't help you out, KiDD, but hopefully it will help someone out.
Well I called the local racing engine machine shop (which is just around the corner from me) and they said it would take about a half hour and cost me 25ish dollars. I borrowed a drill from work and will buy some small EZ outs tomorrow. I didn't loctite these studs in so I hope they will come out easy. Always try the cheapest option first. If that doesn't work I will go to the muffler/welding shop next door to my apartment and try the "tack a bit on and unscrew it with that" trick. I'm just afraid of damaging the threads, I don't want to have to buy a tap or helicoil it.
I just finished sorting this problem out. I didn't have a dremel to cut a notch in the end of the bolt, and couldnt get it out with a centre punch. So I drilled the bolt out, next I filled the whole with a really hard epoxy (same as JB Weld in USA) then tapped an M6 thread into the epoxy and this time put much stronger bolts into the rear mount. I wish I did this before I even mounted my engine, would have saved so much time. I brought the tap set off of eBay for a fiver and the epoxy I already had.