I love projects that can be made mainly with hardware store parts. If you're looking for an easy, affordable, and practical replacement for the crappy stock fuel tank, you can simply use PVC!
Ferball on motorbicycling.com came up with a fuel tank made of PVC pipe and two end caps. I was skeptical at first(since PVC on its own is kinda ugly), but the end product looks great!
UPDATE: If building a PVC fuel tank, I'd recommend applying a proper fuel tank sealer. Some people have claimed success with PVC, but gasoline is known to cause PVC to deteriorate over time. To be on the safe side, use something like Kreem Fuel Tank Liner.