Personally, I haven't had any good experience with getting the cops to do anything about something like this, but yer welcome to try.
Getting a plate number is helpful, but has never been my preference, still, it's not a bad idea.
Catching up to them at the next stoplight, however.....
Be advised that I take no legal responsibility for yer arse over use of any of this advice, since defending one's life and property seems to be some kind of henious "crime" in our society....
(and they wonder how six assholes with boxcutters can hijack an airplane ? cause folks are so damn conditioned NOT to defend themselves...morons...)
Anyhows, here's the best bet - swap your brake handles.
Rear brake on throttle hand, front brake opposite.
This gives you the ability to control yer moped one-handed, leaving your left hand free for something else - 4 feet of medium-weight chain with a lock on the end.
You can "get over" on explaining why it's in your possession as "I use it to chain my 'ped to the bike rack at the mall" etc...providing a handy, logical and believeable excuse for having such a thing on yer person.
And having it many times means NOT having to use it....when someone is cruising up on you with obvious ill intent, pulling out yer chain and givin it a swing or two generally settles matters,...and if not, catching them at the next stoplight may well allow one to settle matters in the only fashion these punks understand.
If you live in a small town, as well....remember what the car looks like.
Punks depend on the greater mass and speed of a car, and count on the socially-conditioned reflex to just knuckle under and keep taking it - you can offset the advantage of greater mass and speed with intelligence and faster reflexes..
(statistics have shown yer average biker responds to traffic conditions over twice as fast as a cage-pusher..)
And as well, once the possibility or even probability, of revenge is introduced, they will stop - word will get around, believe it.