If it's on 6v currently (pun), you could just leave it on 6v. If any of the components have failed, then it might be easier to convert to 12v. If it were me and I was determined to go 12v, I'd either go with an OEM replacement 12v generator and regulator, or fit a 1-wire Delco. Don't know that I'd bother messing around with coil changeouts and such.
I have a 5U that I just restored and it's still 6v and it spins that pony over like it's on steroids. I have the 18A generator which keeps the Optima battery in good shape. I don't have lights yet but am planning to add a front and rear light, in which I'll use LED filaments so the draw is low. The only real update I made was to install a solid state regulator and that was only because the original was rusted through the resistor elements underneath.
The trick with 6v is to have BIG wires (2/0) and to ensure all connections are bright and tight. It's a common misconception in my opinion that 6v is not up to the job. Like any voltage, it's a matter of keeping things in good order. If they are, then 6v works as well as any other. I do realize that there are additional benefits going to 12v such as availability of course. I just like to keep things somewhat original and continuing to run on 6v has some "cool" factor for me. When I get my pinion done (parts at the machinist getting refaced), I'll put up a video of it starting on 6v. It really works well.

Sounds good, Darryl has the part # for the 12v generator. I don't think U-series D2s or D4s were ever 12v - only 6 or 24 at least according to my parts catalog. Looking forward to the thread!