As blue has said if it is a model 2 cylinder than I'd just save the conrod and then you can go from that.If it is a model 1 there is only one way that I know to get it free.Now I can't think of the name of it but there is a mild acid that has to be mixed with water and pour it into the cylinder.Within one to four hours depending on the strength of the acid it will completely remove the rust and allow the piston to be free.Just beware that any acid is dangerous so please take extra care.You might be able to google this.I did it a very long time ago,but I can't remember what the acid was called.
If your not sure as to which model cylinder it is take some pictures from the top as if you were standing over the mower.As well as on an angel from the head side back to the crankcase allowing the fins to be seen,paying particular attention to the smaller fins near the crankcase.Very easy to tell the difference if you know what your looking for.
