the good news is that those bolts are easily accessible and made of a pretty soft material, so removal should be straightforward with the right tools.
If there is enough of the bolt showing, grind a slot for a screwdriver (since the bold head is gone, there should be no tension on the theads anymore) soak the whole thing in PB Blaster at least overnight then try and screw it out.
If that doesn't work, this URL has a nice procedure:
Getting Broken Bolts Out
I have successfully used both the "grind a slot" and easy-out methods to remove sheared bolts. I have also (due to carelessness on my part) had an easy-out break in the pilot hole I drilled and had to have the bolt (and broken off easy-out) removed by a plasma tool at my local machine shop...
Good luck,
Graeme