1) run the hot wire to both filaments. Use the brake switch to ground the stop-lite filament. This way you don't have hot wires run around to the switches to short out. It sounds already configured this way, but best check and be sure.
1.5) Check for a good ground at the taillamp.
2) Go to an autoparts store and get a replacement taillight bulb made with red LEDs. Cost about $10-15, but it is more robust to vibration, brighter, needs less voltage to be bright, and uses almost no current, leaving more left available for the headlamp, and battery charging.
3) The stator must be rewound for adequate power to run a typical DS kit. Perhaps yours wasn't.
4) check if the battery is NiCad or Ni-metal-hydride. If its a sealed lead-acid cell, dump it for the other types, they recharge faster and last longer.
5) check that the headlamp bulb is not over-rated.