gas in oil tank

What brand of generator is it? Who is the manufacturer of the engine? If it's somebody common like Briggs & Stratton, Onan, Generac, etc., there's most likely a manual and parts available for it. If it's a cheap Chinese unit, you're likely out of luck. But that said, the carburetor probably doesn't need to be rebuilt, just taken apart and cleaned. There's not a lot to those little ones so it isn't hard to do. They actually have only a few moving parts.