Mark, I didn't really answer one of your questions, because I haven't seen the bits and can't judge their condition. However I'll tell you what I would do, to get the mower back into operation with a good engine, and a good spare engine backing it up. I'd remove the cylinder head and crankcase cover from the engine with the knock, remove the big end bolts, remove the piston and rod, and inspect the crankpin and the big end bearing in the connecting rod. If I found scoring and wear in the rod but the crankpin was unmarked, I'd reassemble the engine with the connecting rod from the spare engine. I'd use either the original piston or the spare one, whichever was in better condition, and I'd fit one of the sets of new rings. I'd inspect the valve seating and probably lap the valves and adjust the tappet clearance (by swapping the spacer bits or grinding the end of the valve stems). The engine would be back in the mower within two hours. Then at my leisure I'd open up the smoky engine, clean the bits, inspect and measure them, swap pieces from the spare engine to get the best outcome I could, lap the valves, fit new rings, and put it back together as a decently overhauled engine. I'd only throw away worn or scored bits, keeping the rest of the spare engine as future spares.
