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.