The first sounds like a problem. A manual disconnect is fine. That's just for security.
M & H makes engine harness for Year One. If Year One lists it, M & H made it for them.
Why not first see what is damaged. Sure you can haul it here.
Seriously. See waht was damaged. Unwrap the harness. The wire that may be best to scrounge from a good used harnes sor possibly get M&H or Evans to recreate is the one from the alternator. The ring terminal has molded rubber seal over the crimp. I don't know how that was done. At the firewall, there probably is another molded seal.
Anyway you need to see and feel what the damage is. On my car that end of the harness is horn wire, horn power, alternator output, field wire, and temperature sensor. 5 wires. 4 are pretty easy to replicate and 1 of which is more difficult. Your car, I don't know.