If the rings have rusted the cylinder walls it is doubtful you'll get by without a clean up bore job. Try to measure the wear in a cylinder that you got the piston out of to see how much wear you have. If it needs to be bored don't worry about saving the rest of the pistons. The best penetrating oil I have ever used is a 50/50 mix of ATF and paint thinner or acetone. The ATF does not have to be new.
Soak a while, move a little, soak again, move again........it will come out. Don't get in a hurry and ruin something.
