I've used this method before. Here's how it works.
1. If your tank is removable, remove it from the moped.
2. Remove the petcock. Cover the hole with some cellophane and hold it in place with a rubber band.
3. Make sure you have a running hose nearby to rinse off any spillage.
4. Make sure you have a can of WD-40 present.
5. Dump in a litre (quart) of muriatic acid into the empty gas tank. You can buy muriatic acid at any hardware store for a couple of bucks --it's usually in the plumbing or pool section. If you want, add water to the tank as well to dilute the acid (I don't bother).
6. Shake, shake, shake. Slosh the stuff around for anywhere from 5 minutes to 20 minutes depending on how bad the rust is.
7. Remove the cellophane and rubber band and drain the acid/rust from your tank.
8. Rinse out your tank several times with water --you have to work FAST because your tank will begin to rust immediately (within minutes).
9. Put a new (clean) piece of cellophane over the drain hole, and hold it in place with a rubber band. Put about a half-litre (pint) of two-stroke mix into the tank. Slosh it around to coat the inside of the tank.
11. Drain the two-stroke mix out.
12. Fog the inside of the tank with WD-40 to seal the metal from the air.
13. Inspect the inside of the tank --if it's still very rusty go back to step five.
14. Put the tank back on your ped.
15. Fill the tank with two-stroke mix immediately.
Remember to dilute any spilled acid with water immediately. Do not get any acid on your paint or onto anything aluminum (rinse immediately if you do). It's probably a good idea not to get any acid on yourself either.
I've had mixed results using this method. It cleans out the tank quite well, but I find that the rust just comes back again.
I have friends that recommend just taking your tank to a radiator shop and having them seal it with whatever magic concoction they have available. My friend's tank was guaranteed for life.
Hope this helps!