Making your own is not all that difficult, especially if you're just going for functional. It doesn't have to be a harness. Just run the wires from point a to point b following where the originals were. When you're done you can tape or wire tie them into a bundle. As for deciphering the wiring harness really isn't much to it once you figure out what you have. Still have a generator? Magneto or distributor? Regulator or cutout? Which headlight switch? Fuse on dash or back of headlight switch? (If it's a late Cub then you get into 12V and key switches and all that, but a lot fewer of those)
This is a good time to stress good clean connections. A wire brush, sandpaper, and emory cloth are good to have handy to clean up the connections before you put the wire on. A little dielectric grease won't hurt either. If you can do a good strip and crimp on the wire then you should be fine.
With the battery unhooked I would start at the starter and put a big ring terminal on the end of a wire that would fit over the stud. The other end of that wire goes to the ammeter, so stretch out enough wire to reach the ammeter and give yourself some extra in case you have to redo a terminal now or ten years from now
and put on the terminal and make the connection. Since you're at the ammeter put a terminal on the end of another wire and connect it up to the other side of the ammeter. That wire most likely will run to the regulator "bat" terminal so run it up under the gas tank down the right side of the Cub and up to the regulator, leave a little extra, add terminal, shine it up, add dielectric grease, make connection. You just keep going like that.