Dave's right - Genesee Products are great folks to work with. They sell a wiring harness specifically for the GM 10Si alternator you might put on your tractor. Genesee even has a 5/8" pulley for the GM alternator so you can run the normal width belt on the tractor.
AGCO sells a replacement wiring harness like the original. If you get the existing generator re-wired to 12 volt (with the little square regulator sitting on top) for a more original appearance you could use that harness with a little modification - that's what I did on my WD45. Cheaper to buy the alternatopr than to re-wire the generator, but you have to do what suits you.
There are several others that sell harnesses. Duey's website has witing diagrams for all this (link below). For your info, the Genesee wiring harness has a diode built in so you don't have to have the key switch with accessory pole to prevent feedback ....
(Note that Genesee doesn't make electronic ignition for the magneto systems - only for battery ignition systems with 12 v neg ground for AC tractors)