You cannot use a ring magnet in the center of the stock rotor. It looked like a good idea until I saw that each pole of the magnet would be connected through the center. This is a magnet keeper and nullifies the magnetism.
I told this to a guy a few years ago but he went ahead with his project. No magnetic force, no charging.
Now I have thought about using powerful neodymium disk magnets set in the petals of the rotor to start the charging without a battery. The proper pole would need be used on each side of the electromagnet/coil of the rotor. This would give a weak voltage to the rotor to self start it's charging and might give the system a higher overall wattage. I rejected this scheme because the weak voltage probably would not be enough to fire the plugs with a kick start. Also, imagine the carnage when those magnets dislodged and got stuck between teh rotor and stator. ={8^o
Tom Graham