Other than getting ahold of a service manual I have no idea where you'd find a electrical schematic for the machine. Usually the easiest thing to do is remove the old one, make sure you mark ALL of the terminations, and use it for a pattern. That said, having completely rewired several machines over the years I will offer up a few more suggestions for you.
The first is to say check with the mfg and see if they have one available. I say this because a complete rewire is usually a big job. All it takes is to have one wire out of place, too short, etc and it turns the 'pretty', organized, harness you've worked so hard on into a royal PITA to fix because you have to start cutting and splicing, adding, etc to make it right. With an old harness that has already had said 'repairs' done to it it can often be a even larger PITA than I origionally stated it could be...
The second is in line with the reasons I stated that the harness can become a PITA, and that is to mock up/install the new harness before taking the time to wrap it, etc. Doing this will allow you to make sure everything is going to match up like you need it too. One thing for sure, if your harness doesn't run exactly where the old one does, and you use the old one for a pattern, IT WILL NOT MATCH UP ONCE ON THE MACHINE.
Next, everywhere you terminate a wire leave it long, with no terminal on it, until you get the harness otherwise complete and installed in the machine. This way IF things don't work out exactly right with the main part of the harness, you can compentate with the extra wire.
Finally check out the link below to Waytek. They have the correct terminal bodies, terminal ends, wire wraps, and instillation tools to do just about anything you want to do. Even better they have them at prices that are ALOT lower than trying to get the same items from somewhere like NAPA, etc. Granted you may have to buy ten of something you need only one of, due to their minimums, you'll usually get the ten for the same price as you'll get one elsewhere. That said, I may have waited until last to write this suggestion, but it comes down to one really important piece of advice, and that is that if you want to make a difficult job easy, the first thing you need to do is make sure you have the right tools and parts before you start......... Good luck.