You need proper coca-cola not cheap stuff or pepsi as they aren't as corrosive as coca-cola.
Take the spark plugs out and poor a can of coke through each of the spark plug hole's into each bore may take one can for each bore but depends on engine capacity. Put the plugs back in just leave them finger tight and leave for 24 hours or longer and then take plugs out and try turning the engine over with a spanner or a long bar and socket if it's only 24 hours leave for another 24 hours and should be free by then. It does work as i have tried it a long time back and then i rebuilt the engine with new bearing's and piston rings. The coke eats into the rust around the rings and free's them enough to let it move up and down.