I had a tank on my 93 Dr 350 that I was in debate about replacing until a friend told me about Works Toilet Bowl cleaner.  I found them the cheapest at the dollar store.  I used three bottles overall and after all said and done was very impressed with the outcome.  Note take petcock off or it will eat it right off!  Learned the hard way.  Take one bottle and pour it in and shake for 10-15 min, its like a acid and your gonna hear it fizz and make some noise.  It took me three bottles to get the outcome I was pleased with.  After your done rinse out with water throughly and I used a gallon of diesel to treat the metal so the tank wouldn't flash rust.  Cost me $8 or less and looked like a brand new tank.