I had the same problem with the magneto cover until just recently.
What I do to get the Magneto cover off is use the "Inertia Trick".
First of all, get a plain old 3/8ths socket wrench (the most common size) and a hammer. The square head on a 3/8ths wrench fits quite well into the sqare hole, making a more than acceptable substitute. Now, look at the magneto cover, you can see that there's two peices: the main cover and the small central plate (with the square hole). Use a utility knife to make a small scratch mark that runs across the edge where they overlap (this is so you can see if you're making progress and let's you know how close you are to the original position when you put the cover back on).
Now you want to insert your socket wrench into the square hole. KEEP IN MIND: This is a _reverse_ thread and will loosen by turning clockwise not the otehr way around! What you want to do is use the hammer to whack the end of the wrench handle quite hard. Due to inertia, the central plate will (in most cases)be knocked loose a bit (you should see the line you marked split as the two pieces of the cover separate)keep hitting the wrench as needed, but after a few whacks you should be able to simply unscrew the centre plate by hand and take the main cover off (it contains the large magneto magnets, so don't worry about it sticking, just pull away).
When replacing the magneto cover, besure you line up the cover's two small bumps (inside the main cover plate with the magnets) on the two small dents on the hub of the cam.