Does it stay dim even when you rev the motor, you can get a high output stator winding to out in place of the stock one or you can get an upgraded plate with adjustable timing, and high output lighting and ignition coils/windings etc. ,there is even an entire kit mostly for upgraded ignition, but with a more powerful lighting coil as well.
Prices range from around 50 for a stronger coil, to nearly $500 for the complete setup with programmable cdi box , high out put windings, ignition coil, and regulator
of course it could be a weak ground , a short I'm the lighting coil, wrong bulb, weak or poor connections....the list goes on. I haven't been thru the it personally but the service manual does have an electrical trouble shooting guide with test specifications and methods for various components. After checking anything I though might be suspect. I eod look to the service manual next.