Okay, well I'm not an expert on farm equipment, so I'm in need of help here.  My father in law has an old WD45 that he wants to convert over to 12v and he's asked me to help him out with the project as he's a little strapped for time.  He wants to change everything, gauges, lights, etc. so I've got a lot of wiring to do.  I'm thinking we'll use a Chevy alternator since they're cheap and easy to find.  My research tells me that we can use a ballast resistor for the ignition coil, or simply use a 12v coil, but I have no idea what coil would work best.  I'm assuming any 12 automotive coil would do the trick?  The one other thing I need to make sure of, is the starter.  I don't know much about this particular starter, but given its age I'm guessing finding a new one is out of the question.  I understand that as long as he's not cranking on it for long periods of time and giving it time to cool down, the starter should be fine at least for a while.  Since we'll be going to negative ground, does the starter polarity need to be reversed as well or will it still spin properly regardless?

Use a 12 volt coil and a delco 10si, 3 wire alternator (about $50 at autozone).  You can use the original wiring harness.  Just take the wire off the fast charge on the light switch and move it to the on side of the ignition swithch.  Here is a wiring diagram that will help.