Need to isolate problem to fuel, mechinical failure or electricial. 1) try starting fluid to see if engine will start, if it does you could have dirt or water in fuel system and needs removed. 2)check compression if all cylinder check 75-90 lbs mechanical system should be ok but you might want to remove valve cover and make sure all push rod are in place and all valves are opening and closing. 3) you say the electrical system is firing, is the system battery ignition or magneto? Are all caps dry and without cracks?
Are you sure you have spark remember the firing order is 1-2-4-3. Are you the first person to work on it after it quit, maybe someone has removed the magneto and it may be out of time.