I would probably get rid of all the old gas and put new gas in it and put a ounce SEA FOAM in the gas. Get the engine level and crank it over a few time to get some of the new gas and sea foam into the carb. I would go to the hardware store or auto store and buy a new spark plug for it. I would let the motor sit for a day to allow some of the new gas and sea foam to work its way into the engine. Hopefully the motor with new plug and gas in it will start for you the following day. If this doesn't work you may have to individually all the fuel lines and may even have to remove the carb and clean it out.