Sounds like 2 problems:
One: check battery. Bench charge it. Check/tighten connections to it and starter/solenoid. Bypass solenoid with big insulated screwdriver. Still no good? Tear down starter, probably brushes. Can turn commutator in a drill with sand paper.
Two: make Sure of spark --hold plug(s) (All of Them) to head while cranking. See hot sparks?
Ok, then, put them back in, try starting fluid. If it starts, you still have a fuel flow problem., If it doesn't; well, spray starting fluid around carb/manifold boots. Still nothing? You may have jumped timing or lifter(s) broken, etc, stuck valve(s).
If carby problem, make sure a good flow when tap On. If a vac actuated, sucked on the vac line and see. Must Flow, not a dribble--this means bad valve/clogged. Clogged once can be Clogged Again, Nothing is absolute . Remove gas cap--may not be venting.
Check carb bowl(s) Any gas? If not, tap on needle seats. If gas, you still could have clogged idle/slow jests. Any mech that "rebuilt them" would add a $3 inline filter..