Broken Bolt

If the cover is already broken, take a die grinder to the aluminum around the broken stud. You can also use a cold chisel and split the housing to get it off. I don't know if anyone has told you yet or not, but, it's likely you bent the valves on that motor. After you get the timing cover off and change the gears and chain, do a compression test. I would also pull the valve covers and spin the push rods to see if any are bent.