I've seen several cases like this and not one was ever freed to the point the end result was worth the effort. If you really knew the history of the motor you could make an educated decision on moving forward but statistically it sounds like you may be chasing a rainbow.
If its just a valiant effort of spare time you could rig some compressed air to the cylinders in the sealed position above the ports. Unfortunately those are probably not the crusty ones needing the most help.
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[b]The Moose is Loose !
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