Removing the cover is easily done by slipping a thin blade under the pivot/rivet and levering out carefully.
Without a key, the easiest way to remove the lock is to use a punch (3/8" diameter or so) and hammer to drive the lock in. You'll want to have the 'bars turned to where the lock slot is on the steering stem. That breaks off a small screw head and the lock should then come out. There is a risk that the screw head will fall into the lower steering head bearing though.
Better if done while the front end is off. The lock can also be drilled out.
Perhaps a local locksmith can "pick" the lock and make a new key for you?
New lock is available from MG Cycle:
http://www.mgcycle.com/index.php?main_page=product_info&cPath=49&products_id=2950 . I usually buy the cover, rivet and wave washer from a BMW parts vendor.
