The idea with jumpstarting any HD is to charge the battery first and then start normally. Ay battery charger with a 3-amp "trickle charge" feature will work brilliantly.
If you hook in a car battery with the same polarity as your scooter battery, you will have the same voltage but 12.5 times as much current. That can fry your voltage regulator, starter/alternator (deep in the cases) and much more.
Easy does it, and if necessary, replace the battery.