If no one makes a bigger tank, you can have one made. If you watch any of the American Chopper or Biker Build Off shows, you will see that the fabricators are always making gas tanks. They only hold a couple gallons, but you can get a fabricator to make yours bigger. Like they do on TV, you can have it cut and then have pieces welded in to make it taller, or wider, or longer or all of the above.
The best way would be to shape some styrofoam to fit on the tank and see how it looks wider and taller. I think taller would be easier to make. Then get the paint number and decals from Suzuki and have it painted. I saw a Honda Super Hawk from Wyoming that had a larger tank and it looked great.