Fill each cylinder with vinegar if the pistons are stuck. Once the vinegar is inside the crankcase you'll be good! It'll take at least three days. It will also clean the cylinders.
I was thinking, I have some of that stuff that is used to "pickle" galvanized before painting. I can't remember what it's called but it also treats rust and like vinegar, it's a reactive alkaline. Never tried it or vinegar but I can see how either might work.