there is an odd thing i learned years ago
water is the enemy of an engine cylinder for sure,, but once the piston is rusted tight
it can be a welcome friend as well.
if you simply add some clean water on top of the piston and let it set overnight, it will soften the rust
and you can carefully work the crank back and forth and work it free.
this lesson was taught to me by an old timer junk yard guy, that had unstuck many a rusted engine in this manner.
it works,,
if you fill the rusted cylinder with water,, and let it sit for a while you will find the rust will soften to the point that some scotchbrite
or a scouring pad will remove the vast majority of the rust.
this is one of the nice things about good cast iron,, the rust generally only goes so deep and quits,, the outer rusted layer protects the good metal underneath
give it a try, you got nothing to loose
the grease gun method will move the piston as well, but if it is rusted very tight you may break ring lands or pop the top of the piston crown
a 5 inch piston has many sqare inches of area,, about 19 inches or so and if you apply 10k psi, you have available 190,000 lbs of force on the piston
something will move for sure,, but if it is not the piston in whole,, you may end up breaking parts if it is rusted very tight.
i would fill it with water,, scour out all the rust above the piston you can, and let it sit over night,, and then work it back and forth till free
bob g