Does it have overhead valves - check valve adj if it does - easy job. How old is the fuel in it. You may want to remove all of it and replenish with fresh fuel. Check the fuel filter if it has one. I have even removed the air filter and poured a small amount of fuel into the carb to see if I could make it run. Is the fuel turned on?
To check for fire - remove the plug, attach the plug wire to it, lay the plug on the metal part of the engine and pull the cord. Watch for spark. If it is ok then like Benzmac says - it is in the fuel system. Worst case - you may be in for removing/cleaning the carb.