thats because the threads that the bolt is trying to grab are now lying flat.
You need a tap and dye set, you MAY be able to re-cut the threads the same size as they were before but it will be weaker than before too, a tap will cut new threads for the bolt to grab on to. The best fix would be to have it heli-coiled or get a time sert installed http://www.timesert.com/
Those are nuts, You might have to put a little pressure begind them and try to take them out then., If you can get the nuts off, And the cover, Take a pair of vise gripes and back the studs out. Then you can order new studs and nuts, Or get some metric bolts and put in there.