[Dashman]

What I've done for rivets is punch the center of your broken stud, drill an 1/8" pilot hole, switch to 1/4" for about 1/4" deep, then use a hammer drill on hammer mode with a pointed chisel. It should pop right out. The other option is to just keep drilling larger.

You can find individual U-bolts from various online stores. You'll need to measure width, depth, diameter, and thread pitch.