There are a number of gas tank repair kits out there for metal tanks; your problem is the plastic tank.
Quick Googling found this:
http://www.protechnology.com/advants4.htm...that claims to be able to repair plastic gas tanks for motorcycles, etc. It's at least a place to start.
I've also heard of plastic welding that can be done on gas tanks (again, more commmon for motorcycles).
I hope this helps.
