Mark, the bearing shells can easily be pushed out of the connecting rod, either with the point of a knife or sometimes with the tip of a screwdriver. They are only held in place by the slight springiness of the steel backing of the shell, and the small tang at one side of one end of each shell (just above and to the right of your thumb in the last picture). Replacing the connecting rod is unnecessary unless it is defective - normally you might or might not replace the bearing shells when you put an engine back together, but at this point I am not sure I see clear evidence of enough wear on your mower engine's big end bearing to cause a significant noise. The wear pattern on the upper shell of the engine from your mower is assymetrical, indicating a connecting rod misalignment, so it would be worth while using the other rod in that engine. Correcting the misalignment by bending the rod is possible but not recommended for the inexperienced. I also see you have steel rail oil rings, which seldom wear enough to pass oil in any normal lawnmower's lifetime. We should now look for other causes of your knock. One possibility is a loose pulley on the crankshaft extension - the double pulley that drives the blades and rear axle. Another possibility is a problem in the cutter shaft bearings. As I said, without hearing the noise it is a bit difficult. Did the noise occur if you revved the engine with the cutter and wheel drive clutches both disengaged?
