When a magnet comes loose inside the flywheel, it can be Super Glued back in place. But first, be sure to thoroughly clean both the flywheel and magnet (use cleaning solvent or paint thinner to dry out any oily residue), and make sure that the poles of the magnet(s) are in the right postion. North-South-North-South-North-South-North, etc. To find which poles are north and south on the loose magnet, tie a string around the center of the magnet and allow it to dangle in mid-air. The poles will attract to the natural magnetic poles of the Earth. Then write on the magnet which poles are which. Then, when installing the magnet in the flywheel, be sure the poles are opposite of each other. If they attach, it's in the right position.
I was able to get around to fixing my flywheel this morning.
I first cleaned the rust of the flywheel with a wire brush and then roughed it up with some sandpaper. I used a file to clean the back of the magnet and then wiped everything clean with acetone.
The magnets have a slight bevel on one edge making it easy to get it back into its original configuration.
I applied a thin layer of JB Weld to the back of the magnet, clamped it and made sure it was aligned properly.
I think that I would put a bit of paint onto the side of that magnet just so I knew down the road, that was the one I refastened. Maybe you will never have a magnet come loose again but if you do, it would be nice to know to a certainty that it was not the one you dealt with previously.