If you are doing a true restoration it should have a six volt system but a genny, cut out, and a battery are going to set you back several hundred dollars. If you are actually going to use it keep the 12v. It's just my opinion but I woulden't give a plug nickel for any six volt worker. I've pushed, pulled and rolled too many of them back when I was young enough to do it.
You didn't say whether you have an alt or a genny? Also what is wrong that makes you think it isn't a good conversion?
If you do a search on johnnypopper I have the original 6v diagram on there as well as the 12v three wire Delco conversion I did on my M. I converted it about 30 years ago. There are several way's to do it and it's pretty simple. You take the original field wire and remove it from the switch forward. Then put a larger wire in place of the gen one and go to the alt with it. You can feed the alt plug from the big post on the back & use a diode to the No1 term or run it through an oil pressure switch like I did mine.