If that engine is as rare as indicated, haul it out to an engine stand and disassemble it carefully. Yes, I would continue to soak it fully with something like Kroil/ATF/WD-40 to try to break it loose, but my guess is that the iron rings may be rust frozen solidly to the cylinder walls - and without knowing what this engine is manufactured from - aluminum with iron cylinder bore liners (most likely), you may have an entire engine full of fluffy white crap which is oxidized aluminum. Yanking and twisting may damage it beyond any reasonable economic repairs and hope - after 30 years of neglect.