No, the harness for the 56 Bigtwin is not available for purchase, very tough to find a decent used one as well.
That being said, it wouldn’t be too tough to make up a new electric harness for the engine if you are handy that way. You could probably use a harness from a later OMC engine, and mount the solenoid to the engine pan.
Again, this engine uses a 6 volt battery, but could be converted to 12 volts without too much trouble.
I guess what it comes down to is how much you like the 56 Bigtwin, and if it is in good enough mechanical condition to warrant the extra electrical engineering. This engine would certainly look period correct on your nice rig.
One final and important consideration. What is the transom height on your boat? Please be sure you get the proper shaft length engine to match your boat!
