I would mix diesel fuel, PB Nutblaster, Deep Creep, and Marvel Mystery oil all in one, and dut it down the cylinder and maybe a little down the intake. We call diesel the magical fluid, as it will clean or free up about anything. Deep Creep and PB Nutblaster really do work. Prices are a little high for them, but good quality for freeing rusted things. WD-40 is ok, but really doesn't free anything rusted up much.....
Maybe you could take the heads off, take a torch, and try heating up the pistons to cook the rust. Thats what we did on a 1950 JD model...........and we also used a 20 ton jack on the pistons
