I'd just drill it out from the front side.. outward side which will back off the bolt(generate a loosening force as you drill). start with something like 3/16, 1/4 and try your best to center punch the bolt and get close to the actual center. If it doesn't come out by the time you get close to the threads.. use an easy out. I would soak it for a day with Kroil before drilling too.

For the replacement, dealer works if you have one close. Heads on all metric hex bolts are marked the strengths in Mega pascals or maybe it's kilo. 8.8 , 10.9, 12.9 etc.
So get the right length(measure the other one) and they are just course thread bolts- 10.9 would be my guess. Fastenal, acehardware etc.. should run you a couple bucks for a pair. I'd replace all of them both sides. Install with blue loctite to prevent corrosion in the threads and failure that you just experienced.

Good luck!
Michael