Been there, done that, many times in bikes and cars.  I use this kit:

http://www.hirschauto.com/prodinfo.asp?number=CRK-01

This kit has everything you need for $30.00.  I used their automobile gas tank kit on a internally rusted car tank and have had no problems with the tank for 11 years now.  Hirsch has been one of the leaders in automobile restoration products, for many years.  It is important that the sealer you use inside the tank withstands all of the additives and alcohol blends, that gasoline has these days.  Not every tank sealer does.

What is also important is that you seal the tank after you remove the rust, otherwise it will start again as a result of the moisture in your gas, and the surrounding air inside the tank.