Get a spark checker and make sure you are getting spark, I'm new don't have a clue about 1979 20hp. We have a 125hp it's test coil if you have a coil. Test trigger if you have it, test stator if you have it, make sure all connections are clean and tight.
You said it was running a few day ago,was this before you did all the work on the motor? If your positive you have gas, its spark, put your glasses on take the cover off and tug and twist every wire ,connector, check every inch of wire. Inside of motor is fine, something outside the motor is broken , find it.
I would get new points, condenser, rotor? that part that spins and touch the cap contacts and any other cheap ignition parts I can't think of, clean the contacts on the cap,and any other parts in the distributer.
Please tell us what it was so I can learn something.