last case dremel, or drill it right thru and helical kit, or force big bolt in there that usually hold, better yet=new crankcase or motor
from my 4 years of motorize biking i have had snapped studs 4 times, (now use high tensile haha) easy outs were for me a big fail (i used cheap ones etc so may be my luck) the other 2 i dremeled a grove and used a flat head screwdriver (barely even marked the crankcase) The last i nothing worked as i used red loctite lol so i drilled it at 7mm then used a drill bit with a thread (like a rough threader bit) thin tightened a bolt into it (one that matches the rough thread job) still works after over a year fine. i guess my suggestion o new case or motor was pretty stupid, so apologies for that. if not take to professionals i guess?
another PRO way is to weld a washer to the stud and then a nut to the washer.
And this works with broken easyouts and drillbits if not busted to far off down in the hole
Don't waste you time. Drill two holes thru the mount 90 degrees to the original studs and then bolt and attach angle iron or a steel "L" to it for a new engine mount...much more strdy
What has worked for me a couple of times was just blocking the motor in my little drill press and after running a bolt that size up to hardware store and bought a brand new drill bit and easy-out, it worked like a champ but the trick as mentioned is drill the stud and not letting the bit slip into the cheap soft case metal.
One other note is unless you snapped it off tightening it your motor had to be loose to snap it off. One simple test you should do before you take a ride is grab the top of your motor with one hand, your top bar just behind the gas tank and try to push you motor back and forth, if it moves AT ALL it's need tightening.