Uh oh...voltage crash and now won't turn over

Agree, I would guess at this point the battery is dead

If "13" is an accurate reading even that is low. A proper "low to medium" cruise RPM with a normalized/ charged battery should run nominal 14

Are you still running the original style altlernator/ regulator? That is the grounded field / one field connection?

If so, monitor battery voltage, disconnect the field wire, and clip the alternator field over to a 12V source, such as the starter relay stud. Then slowly bring up RPM and see if the voltage comes up