the carb should be held on by a nut and bolt, mine is anyway. you should be able to get a wrench in there. it takes a while since you can only make about an 1/8th of a turn each time, but it will happen. it that doesnt work just take the intake off at the cylinder.
you should be able to find a rubber gasket to fit at an autoparts store or a hardware store. check your float and make sure it isnt stuck or upside down like miker said. that should be it assuming it is your carb and not your petcock.