just go to your local auto store and pick up a bolt removal kit
you're REALLY lucky that you snapped one on the front cover, otherwise you'd be screwed.
the best way to do it is by taking everyghing off again (cover, gasket, etc.), and then just drilling it out.
I snapped two on my front cover when I did mine and they came out no problem. If you want to be on the safe side, go pick up ALL new valve cover bolts for the reinstall. They use crush washers and after being on the car for 10yeras, they don't crush very well anymore and the bolt itself snaps
good luck