Is there a larger gas tank I can put on my 2002 xl1200 sportster sport.

Some of the aftermarket manufacturers make what they call a "king tank". I don't know how much more fuel that will give you if any at all. that's the biggest tank a Sporty that I've seen. Now, I have seen a set of Fat Boy tanks on a Sportster and they look good. Of course, this would require that you fabricate a custom mount for the tanks.