How about one of those wagner paint sprayers--- hook it up so it's a rear mount. Anyone gives you trouble give them a new paint job.
Or get a small tank of acetone, and link it to a windshield washer pump, spray people in the eyes, or melt the paint off the SUV.
You can make smokescreens by modifying the muffler to accept straight oil into the hottest part of the pipe.
A spring loaded box, containing "jacks" you've welded together from roofing nails. About 100 of these should stop a tailgater.
I've used a watter bottle filled with fish juice from the wharf as well. a face full of that will get anyone pretty upset.
a rear-mounted flamethrower is also fairly easy to make. There are plans all over the wed (they say you're supposed to use it to kill bees nests way up in trees.
Then, you could always carry a cell phone, and call the police and charge the person yourself with reckless driving.
A small air compressor could also be used to fire projectiles at other drivers.
You could rig up the windshield washer pump, connect it to a bottle of piss or sour milk, and make sure the sprayer is aimed at the driver side window of the offending vehicle. Wait for a stop light, and spray them in the face with some kind of medical waste.
Or, just keep getting in front of the same person at every redlight. they'll get mad and pass you, and at the next red light, pull in right in front of them again. repeat.
I also use airhorns. dual airhorns on a moped tend to scare the life out of people. Catch the dude who nearly killed you at a stop light and blast him through his window. Sooooo loud.
also, it seems like a fairly decent sling shot could be added to the fron end. You couldn't use it while driving (unless you're really good....) but a 4 oz ball bearing can have quite an effect on a side panel of a vehicle.
Further, if you were to get a large tesla coil, and mount it on the back of the moped, you could desttroy the electrical system of any car you wanted. You'll need to surround your own moped with a protective 'farraday' cage, but the gigantic blue bolts of electricity shooting from your moped should be worth the trouble.
I've been working on a torsion field generator, as well as an M.E.G. that extracts usable energy from the 4th dimension (time). There's a ton of energy bound up in the timecycle, you just cant acces it with our current "bipolar" electrical systems. You need a continious 'dipole' to extract energy from the quantum vacuume.
Or, just mount a P.A. System on your moped,so you can taunt the drivers who offend you. "....your mother was a hampster, and your father smelt of elderberries....."
Good luck. I'll post some info about how to use red mercury to create portable thermonuclear devices for fun and profit.
I usually put a tiny atomic bomb in my nose and then when friends are around, I pretend to have a huge sneeze coming on.....
Ahhhhh......ahhhhh...aaaachooooo (that's when I hit the trigger....)
what a blast!! (no pun intended)!