Some of your part names appear to be a little off. On bikes, rotor usually refers to a brake disc.
I think you are talking about the magnets on the inside of the flywheel breaking off. This is common on older KDX's.
Underneath the flyheel is a lighting coil as well as the stator. The stator is part of the ignition system, and stator failure is also common on older KDX's and can result in it being impossible to start the bike. The old stator can be rewound by a company like Ricky Stator.
The '86 - '88 KDX200 should share the same parts. I'm not sure if an older flywheel from an '83 - '85 would work. You could try calling Jeff Fredette at Fredette Racing Products and ask him, as he will know for sure. Perhaps he even has a used flywheel to sell.