Hopefully its' the petcock seal or the float bowl gasket. Easy stuff. The petcock nut is a combination right and left thread, check the manual for messing with it.
The tank takes a bend horizontally for a bit, going above the motor. That is the very bottom of the tank. Check by taking off the air filter and looking or swabbing into the round hole in the side of the frame for gas. It may be hard to acess that area for patching but if the tank is rusty that may be a tell tale sign of a weak frame.
Hopefully not, but Marine Tex for patching over clean bare steel on the outside, with Kreem or other tank sealer for the inside, following the directions to the tee.