First thing that I'd check is to see if it's getting fuel. Check the fuel filter as well. You'll need to narrow down if it's a spark issue or a fuel issue. Fuel issue= take hose off at carb or if you can see filter and it's a clear type, look to see if fuel is moving while someone try's to start it. Spark issue= Use a pair of pliers with rubber or insulated handles and pull off the spark plug wire and hold it about 1/4 inch off of the head or block. Have someone crank it and see if spark jumps across.
See if one of those are the issue. Good luck.