That Aint a Wire, Thats a Ground Strap.
Refit it, or better yet, replace it with something less bare braided metal. You can find Grounding Straps in various lengths, sometimes for Surplus and/or Welding Shops.
The Strap in question went to the Firewall, or Bulkhead, usually to the Nut holding the Coil in it's bracket, (right?)
It's important to properly tie the Motor, Chassis, Battery and Alt's Case together, electrically speaking.
While we're on the subject, you might also consider replacing the Ground Strap from the Battery to the Chassis and Motor/Block as well.
(It's a three point connection so you might find it easier to replace it with two conventional straps; one w/ a Battery Clamp on one end and the other cable being for bolt attachments on both ends.)
A good, medium gauge wire from the Alternator's Case (not the electrical output posts/blades, but the Case or shell) to the Engine Block (sometimes folks use an additional nut to a valve cover bolt, at least on 8v motors).
Aaaand, of course the Hot Side of the Battery/Alt will be jealous of all the attention so a good gauge Cable from Battery to Alt Hot 12v is also recommended. (Some Alts have blade connectors, some have a bolt/nut post...)
Last thing, get the mating surfaces clean and shiny brite and slap a thin coating of something on them prior to the rebolt to keep the Oxy-dation out going forward; even Petro-jelly is better than nothing.
as an example...