I have a 1976 Dt400c I dealt with the regulator by changing to a 12v system with a SLA sealed lead acid battery that fit in the oil tank pocket.

Trail tech sells a full wave regulator rectifier for $59.99 and is great if you want to convert to a 12v system

This can be done by pulling flywheel, and completely isolating the 6volt lighting coil from the frame. You do this by cutting the wire from the lighting coil that grounds to the stator plate and connecting both ends of the coil to two independent wires that then go to the trailtech full wave regulator rectifier which in turn regulates the charging voltage and feeds the battery. 

People call this "Floating the Ground" but took me 3 months to find an explaination of how you float the the ground. 

One Caveat is that trail tech says in their instructions that you need to separate your 12v dc wiring from the frame, meaning each light needs to ground to the battery and not through the frame which is still running the AC powered ignition system. This does mean a bit more work running negative wires to your lights and isolating the light sockets from the frame

This is one solution, and allows you to run 12v led lighting which consumes on my bike 32watts total with an amazing amount of light for night time trail riding

Best of luck to you
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