Many places will sell you a float needle that fits if you bring them the old one. Likewise the float itsself is usually still available.
To replace it is fairly simple. You just take off your air filter box, the metal shroud over the engine (if you have yours) and disconnect everything from the carburator. Then unbolt the carburator, unscrew the bottom half of it and there's your float and float needle.
If you unscrew the 1 screw holding the float and needle in place, you can clean the area real well to rule gunk out.
Then hold the float itsself in a cup of water, turning it around a few times. Pull it out and look at it up to a light/shake it next to your ear to see if there's water in it. If so, it's time to replace.
If it's fine though, you're needle has probably just worn and isnt sealing any more. A whole $2 item at a lot of places.
Of course before you do all this I'd ask just where the gas is leaking from. If it's pouring out the overflow tube that runs down the front of the transmission, then I'm guessing the float's the problem. But if it's leaking down the sides of the carburator I'd suggest replacing the bowl gasket.