i too would suspect the valves first. i have freed up many old and i mean old tractor engines i tap the rockers with a ball peen
hammer a solid sound means the cam is lifting the valve a ping sound means a good loose valve and a thud sound is a stuck valve. i soak my engines with a mixture of oil be it marvel or just motor oil and some sort of solvent acetone or paint reducer once i get it to move i go back and forth a little at a time until i get a full rotation this helps prevent crud from breaking the rings. if it has just been sitting and no rain in the engined an old rope seal engine will run again if you are persistent.
