1) I had a gas tank that leaked at one of the mounting bolts. I fixed it by getting a fender washer (large outer diameter, little hole), a rubber hose washer that matched it's size, and some fuel proof sealant. Used the sealant to "glue" the rubber washer to the other one and then filled the inside with sealant. Then I used a nut to hold this against the tank to seal the leak. Worked like a charm. I'd have just replaced the tank but I had something painted on it that I didn't want to have to repaint. If you have leaks in more than one spot, I'd replace the tank.
2) Buy a metal exhaust gasket. Sick Bike Parts sells an aluminum one
3) This is a good time to upgrade your exhaust, but the stock original ones go for about $15-$25 from many of the vendors who are sponsors of this site.