The expensive but best penetrating stuff I have found is Wurth Rostoff. This tends to be a trade product so you might need to get a chum with a garage to buy it. It is 10 times as good as things like WD40. It is more expensive and I keep it for special situations. Ultimate attack is a day or so of working Wurth in then into the vat of diesel for a few weeks. Most things move after that, though really noshed piston/barrels and such where there is electrolytic corrosion and/or damage are often to resistant to move without force.
The penetrating oil that I have immense success with is Kano-Lab's Kroil. Expensive again, but just a few drops, a little tap in an hour, and things come loose. If they do not then repeat again. It is amazing. If it doesn't work then the heat is the only alternative. Never tried it on pistons/rings, but may work. Even on surface rust if it sits, then with some wet/dry 600, things sparkle.
AC, Acoma, Argsons, Arola, Batricars, Bianchina, Carter, Citroens, Cushmans, Dafs, ElectricPal, ElectricShopper, Elswick, Eshelman, Everest, Goggo, Harding, Invacar, Levesons, Lloyd, Marketeer, Model 70's (AC, Invacar), Nelcos, Poirier, Reliants, Renaults, Sinclairs,Trabant, Trilox, Tippen etal