can you turn that motor over with a socket?

I would seriously consider pulling the heads. the intake is off, so all you need to do now is pull 10 bolts out of each head, and new head gaskets for $40.

chances are, no one backed the rockers off, so some valves were held open all those years. a mouse may have found a nice little place to build a nest. that'll be hard to tell for sure without pulling the head, unless you have a bore scope.

once you verify it's clean in the bores, pour some kerosene or other environmentally friendly solvent that they let you have over there, that is NOT water based. (gasoline will work if that's all you can get, but BE CAREFUL!!!) then fill it up with fresh oil, prime the oil pump and let it rip!