There's a few things you might try, none of which may work. I think I'd start from the least possibly damaging and move toward things with a greater possibility of damaging parts. The first thing I would have tried is looking into the cylinder through the spark plug hole with a bore scope to see how much rust/corrosion was visible but since there's already been WD-40 & MM Oil in there a scope may not reveal much at this point. You might try adding some kerosene to the MM oil already in the cylinder. Kerosene is quite thin (low viscosity) with little surface tension and can work it's way into small places carrying oil with it. Let is sit a few days and try turning with a breaker bar. You may be able to pump HOT water through the water jackets which would expand the cylinder diameter slightly and allow oil/kerosene to reach even more areas. Try turning it over while hot. Sometimes rocking back and forth (clockwise & counterclockwise) can break things free. If none of that worked you might try applying air pressure to the cylinder if you are lucky enough to be in a portion of the cycle to have both valves closed and the piston not at exact top dead center. High air pressure may force it over and at the very least may force some lube to areas between the piston, rings and cylinder wall that haven't yet received any. If there was moisture in the cylinder it's likely the rings have rusted. When steel or cast iron rust they expand reducing clearance sticking things in place. You might try pouring some Evap O Rust into the cylinder in an attempt to dissolve any rust but with all that oil in there it may no longer be able to reach the areas it needs to reach. Sometimes a little shock or vibration can be helpful. Gentle use of an impact wrench to try to turn it over can be attempted but don't get carried away. It's possible to break crankshafts or strip fasteners you are using to attempt rotation ultimately destroying an otherwise good part.
In any case, if things are rusted/corroded that bad it's highly likely that you'll need to do a top end job before it will ever run well.
In any case, if things are rusted/corroded that bad it's highly likely that you'll need to do a top end job before it will ever run well.
