Pull it off, drain it, take it to a body shop or a welder, and have it welded. Don't try any "fix it" crap, because it diesn't work. They will most likely have to fill it w/water, do the sanding and stripping, then the welding. It depends on how bad the cancer is on the inside whether it is worth repairing.
You might want to hit eBay for a spare while this is being done, or else just get a nice one off eBay.
If it was a 1st gen, there is a way to mod the tank to get an extra gallon of gas if you are not using the airbox. I think you can do it to your model too.
Start doing your research. Start with this book:
Amazon still has them. The ultimate 80's Sportbike Prep book by Jewel Hendricks. I've had mine almost since it came out.
ISBN: 0879383011
- Nut