It's not too hard to make your own. 14 to 18 gauge multi strand wire is available in most automotive stores, tho in limited colors. British Wiring can supply wire in many original color schemes. If you are not worried about stock appearance, you can use your imagination for alternatives to the bullet connectors. From Weatherhead connectors to simple spades.
A simplified diagram was printed in the original USNOA tech digest, but if you study the diagram from the factory manual, you can easily figure out how to eliminate a couple miles of voltage eating wire.
Adding a small fuse box with blade type fuses and separately fusing the ignition, lighting, and horn circuits makes sense at this point also. As does using small automotive relays to take the current draw away from the Lucas handlebar switches. With a little thought and fabrication, it can all fit in the original battery area.
I have done this on my 850 Interstate and along with a Podtronics regulator/rectifier and three phase alternator, has made a huge improvement in electrical reliability.