condensation can still cause a lot of rust - but it's conceivable that you can break it free.
fill the cylinders till it leaks out the spark plug holes. Then stick an air hose in and blast the fluid so it bubbles and splashes all over everything inside there.
Do that often -
the penetrating oil will do its work, the parts don't necessarily have to bathe in it. As long as you constantly wet them like that.
Put it in the highest gear and keep rocking it back and forth, kind of hard.
Keep an eye on the flywheel. When you lose faith and feel it's never going to move, you might just see a little jump. That'll keep you going till it fully pops free.
for something untouched for that long, I'd definitely avoid pulling with another tractor, or something more extreme. Better to operate first.
Good luck - keep us updated.