Most auto electric shops can do this for you. I had mine done for around $65 but that was 15 years ago so it might run a bit higher now. I don't know the process but they could probably explain it to you.
You didnt say what brand you are working with. I had a 48 Ford generator I wanted to make 12 volts. I took it and a generator off a 56 F6 to the local pro. He put the two together, and it has been flawless for several years.
Yes, there are plenty of shops that do it. I think there's an alliance member that does it. I believe all you do is switch out the field coils and use a 12 volt regulator.
This has been covered before. Find more details with the "Search" function. Cheapest route, change the 6 volt regulator to a 12 volt regulator. Second option, change the regulator and also change the fields to those from a 12 volt generator. I have done both and both work, at least with Fords. Again, do a seach and do some reading. That is how I found out how to do it. Neal After you do a search and still have questions, shoot me a PM. I will try to answer your question.