The magnets are glued to the inside of the flywheel from the factory. After 20 - 30 years, the glue is worn out, and the magnet will fall off when the engine is running, and with the spinning force, will destroy the stator assembly, and possibly jam up and lock up the motor at the crank. This happened to mine one time a long time ago. My 84 rear frame near grab bar was destroyed when I picked it up, and had been mended back together with rebar, but I never noticed any cracks near the footpegs.
THE 80's RAGE! Currently owns:
81, 85 Yamaha Tri Moto 125
83 Yamaha Tri Moto 200 x2
82,83 Yamaha Tri Moto 175 project
85 Yamaha Tri Z 250
84 Kawasaki Tecate 250
84 Honda ATC 70
85 Honda ATC 350X
84 Honda ATC 250R
(2) 84 Honda ATC 200x
82, 83 Honda ATC 185s
81 ATC 200
Lots of other parts and bikes