Craigore, I'm assuming Carbie Bonneville or t-100 here. generally speaking the main harness from the bonneville, t-100, and Thruxton are similar enough to work, especially if you are going to chop it up anyway. If you find an early Bonneville harness it won't have the red Tach wire. I personally shied away from the standard bonneville harness, and used a Thruxton harness on my T-100 which worked perfectly until it was eaten by mice (little fuckers) and am now using a T-100 harness. Got both those harnesses used from Pinwall. Best of luck with the search, if you wait it out and are pretty vigilant about looking one will come up at pinwalls and they are the only vendor that has cheap main harnesses on ebay.
I don't think an America/SM harness would work easily...