Personally - I'd stop riding the bike immediately, or even running it for that matter. All else aside you'd just sheer off the one remaining exhaust stud.
As bairdco mentioned, if you're very,
very lucky there'll be enough bolt left to grab with some vice grips - if not, it's extracting time... which is a bit of an adventure.
Using so called "EZ outs" is not exactly easy. If you haven't done it before or even seen it done and don't have the tools... I almost wonder if it wouldn't be worth it to take it to your local auto shop and give 'em a few bucks to do it for you - while you watch ofc.
I only say this because judging from your questions you may not be used to tinkering with engines and a broken off flush stud can be a pain even for the most experienced. One of my favorite tricks is to take a nut and place it over the broken stud and weld the two together, removing the "bolt" I just made while it's still hot.
Of course you'd need access to welding equipment for that...
If ya wanna try it yourself, you drill a small hole in the bolt you're trying to get out, then screw an EZ out in counterclockwise. When it gets in far enough, it will cause the fastener to start screwing out... usually. As yours broke because of sheering load it'd prolly work just fine - it's when they break while you where tryin' to unscrew them that there's problems as the threads are "seized".
EZ outs/bolt extractors look like this and should be available at your local hardware/auto parts store;
Good luck
