The harness should be the same from 1971-1974 750 and 850. Available from Old Britts or British Wiring and I'm sure others.
Also easy to make your own from components from British Wiring. If you aren't building a concours restoration and interested in electrical upgrades (That is, exorcise the Prince of Darkness) then you can simplify the harness as the original has quite a few wires you don't use unless you are running Police lights, radio, and siren.
British Wiring also sells whole harnesses, ready to install. They don't include any of the Interpol wiring so it's already significantly simplified. If you're installing an electronic ignition it'll still have some extra wires but you can easily tape them off or cut them off where they come out of the harness wrapping.
I think their part # for your bike would be MC32 but double check with them. Keep in mind you'll also need a headlight harness if you don't have one already.
I have one of their harnesses and it works fine. The only problem I had was some of the connectors are spades where my equipment used bullets and vice-versa, but in that case I just cut off both sides and used Posi-locks. Sturdier than either bullets or spades and reusable.