The bulb is like a resistor so low voltage will just give a dimmer light. You need a volt meter (Lowes, Ace, Auto Zone sell these inexpensive little units that should serve your needs for now). With that you can make sure those brown wires have juice coming out of them (using the volt meter you can ground it right at the battery to by pass any grounding issues).
With the volt meter you can test where the voltage stops, if the switch works; useful information in tracing things out like this. A simple continuity checker, basically a 12v light bulb with two leads would also work but will not reveal your voltage which would answer your low voltage question.
When I installed a couple Vetters back in the day, and serviced some bikes with Vetters, the wiring could be very iffy. Not exactly state of the art and if it was owner installed, an owner with few skills, all bets are off on what was done to get the light to work.
With my Moto Guzzi Quota we used relays to run battery current directly to the headlight with a low current light duty wire to the switch so we could maximize the light produced by the bulb. Quota's are notorious for very poor lighting; this is something to consider in your application too. I have no feel for how much power the 750 will produce but back in the day there was little need to give a motorcycle a big generator until Goldwings came out and folks wanted to light them up like diesel tractor trailers.
Painted light buckets could indeed cause you poor grounding. It would not be a bad idea to run a ground wire from the frame into the bucket to provide a ground path while keeping the paint intact.
