I also just purchased a used D & B 5.1. If the unshielded plug leads are not a problem, what's the reason for changing to a 6 volt ignition? 4.8 will deliver enough volts to fire the engine adequately.
I was more interested if there might be a weight loss with the newer style of ignition units.
Either way I think these are one nice engine for sure. I expect to fire mine up real soon.
Darrolair