If the spark is there there is no problem with the ignition system, there are no points and timing can't be out as you cannot adjust it on the 4hp, it has a fixed timing and ignition curve.
99% of the time it is the carb at fault. Go through it again. Check jets, low speed one screws into the side of carb easy to miss if you don't know it is there. Diaphragms definatly the right way round with the little cup washer and spring in position? Spring fits into recess in fuel pump casting and cup washer fits onto spring and presses against diaphragm.
Compression? Without a compression gauge you should be able to get a good feel for it pulling it over. At 30years old the crankseals may be starting to go which does affect starting.
Far better engine than anything four stroke on the market these days.