You want a NEW wiring harness? This is the only place you can get one new.
Antique and Classic Outboard Motor Paint & Decals
If you want used, place a wanted ad in your local classifieds, or at aomci.org in the webvertize. They are hard to find though in good condition. I have 2, and the only way you would get them from me is after my funeral from my wife
Google "Hildebrand Marine" in Coldwater, MI. They have tons of old vintage hard to find parts, etc. I only live 30 miles away and just converted my 72 18hp Evinrude to electric start. He had the mount, starter, solenoid, cables switches everything. I'd bet he'd have what you need. I think it's Seadoo clinic - Snowmobile cylinders, Personal Watercraft, and Outboard Motors. Check it out.