Well, this engine is a tough one because it has a specific type of main connector that is more or less unique to just a few years of engines, so the harness can be tough to find. You will notice that your engine has a black main engine connector. The common version of this style of wiring uses a red main connector.
I don’t recommend trying to make a new harness, because how would you duplicate the black main connector? Do you have the boat wiring harness, is it in good condition? If so, you have only a few choices: look for a used engine harness or perhaps buy a new one from CDI.
Another choice would be to just eliminate the black engine connector and hard wire the boat wiring harness to the engine, but first make sure there is enough extra harness in the boat so it can be pulled farther into the engine. Simply cut the black connector off the engine harness, peel back a few feet of the outer insulation, and hard wire the various leads to the engine.
One other alternative would be to find the much more common red plug boat harness, and an engine harness from just about any OMC engine that uses the red plug. Now simply cut (or extend) the various leads to a length that will fit the terminals/connections on the engine. This will update the engine wiring to a more common version while allowing the engine to be removed/installed without disconnecting all the various leads.
I don’t have a 79 manual, but I’m sure someone will post a wiring diagram with color codes.

Seloc says that wiring diagram applies to 20-40 hp engines ’79-’85. The diagram I have in the official service manual is too big for my scanner. If you want the manual, send me a PM (available cheaply).