Not to dredge up an insanely old thread, but I just picked one of these up off of E-Bay and have been working on restoring it. I've got the lenses back in place and its all looking good, so sorting the wiring out now.
The voltage (and some other useful info) I was able to track down in an old Popular Mechanics ad for surplus sights. It looks like it takes anywhere from 14-22 volts (the lightbulbs seem to have been available in multiple voltages for this reason):
It says AC or DC which I find a little confusing, as I didn't think anything could be "either"
The same magazine has surplus Rolls Royce Merlin V12s for $200 new in the crate, THAT has all sorts of potential for fun (assuming they still honor the pricing ).