This was a common thing to do for 6v VWs. All you had to do was change the generator+regulator (which I guess you call dynamo?) to a 12v generator, make certain all your bulbs everywhere (headlight, turn sigs, dash lights, etc) were changed to 12v and you were good to go. The switches did not know any difference with 6v or 12v, so no need to change them. If you were lazy or smart or cheap (take your pick), you just left the starter alone. It spun faster, but you only needed to give it a very quick shot and it started the VW 1600 engine easily. I did that with my VW and the starter was still good after 30 years that I owned my VW. If you only have "kick-start", then that is not an issue.
Reversing to a negative ground is pretty easy, it takes some little bits of copper here and there - just cannibalize some from old trailer lights or turn signals and make your own pieces. I had to do this to put LED bulbs in my turn signals. It was tedious, but doable. If you are doing all the wire over again, I imagine it may even be easier.
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