Glad you got the bolts out

Very important that you take the stainless out and replace even if you have to use Home Depot. Stainless is a weak metal and will hold less than any steel bolt. The worst part is stainless will gall or smear to where removal if they break will be a nightmare, drilling stainless can also be a problem. Best use a 10.9 grade metric bolt but even and 8.8 grade would be much better than any stainless.

For 14 years I taught Diesel Mechanics at a Community College and taught a basic shop class that included removal of broken bolts, I would twist off bolts in an old engine block and have students remove them. Best to try in the following order:
1 if enough is exposed grip with tool and remove
2 try to tap it around with chisel or punch
3 weld on a nut (don't really recommend this if broken bellow surface) if you screw up you weld the nut to frame
4 drill for easyout (best if you have left hand drill as it might come out) make sure you drill in the exact center.
5 drill to the threads and re-tap, if you did not drill in the exact center for # 4 you will need to use a die grinder to try to recenter hole, not fun
6 setup on milling machine and oversize for insert
7 buy a new bike/engine block/or what ever

I broke all six bolts holding my boxes on my South American trip, finally oversized with inserts (not helacoils). good luck

Bob