Honda still sells the K0-K1 harness. For some reason..
For K3-6 you'll have to go aftermarket. (Or pay a king's ransom for a genuine NOS harness.)
That harness from VintageCB750 is the same one Partsnmore sells.
I have one of those harnesses in my K5 installed by the PO.
Harness looks good, proper gauge wire, good connectors.
I have had no issues.