Hi Martin
I have an 80 Plus so dont know about the magneto on the 'Fly however I did convert to 12V which in my case was as easy as a new battery, new regulator (12V of course) and 12V globes.
The advantages I think I get from 12V include:
1. Better, brighter lights, I have a sealed beam on the front.
2 More Watt/Hours from a battery of the same physical size
3 Louder horn
4 Less wiring losses through lower current
and the big one
5 Easy to buy the bits!