You are correct, I have one 1961 manual shift Johnson 75 in extremely good condition, that came on my 1960 Cruisers 202. I have since purchased three 1960 Evinrude 75's. One is in great shape but less than my Johnson, and the other 2 are restorable. All mine have original harnesses, good thing is the 1960 75hp and 40 hp use the same harness, easy to swap engines. 75 runs a lot faster but burns 50% more gas at 20 mph, so I run the 40 the most.
Are you talking about the harness from dash to engine or internal engine.
Wiring harness starting in 1961 would be very easy, just wire and some sort of sheathing around wire. Match wire size and you should be good.
The wiring harness used with 1960 and before are harder, as they have a built in disconnect (plug).
I did replace a harness under the engine cover on a friends 1977 35 hp Evinrude last summer. Not cheap, but still available from my Evinrude dealer. List was about $180.
Lots of old engines around with good wiring harnesses, try placing an wanted ad for one at
www.aomci.org .