Rich, I suggest you take a bunch of pictures before you put it back together. We probably won't learn much from what it looks like assembled.
The failures for which the K46 is notable usually seem to be hydraulic failures, not gear or shaft failures. It may be that the transmission's hydraulics do not usually last long enough for the gears to fail. I am doubtful that an internal hydraulic failure would happen with a bang, though, unless a pipe or hose burst or a gasket blew. Typically the hydraulics wear out, creating leakage paths, and the transmission is then unable to pump enough oil to generate much torque. However any leakage path, including a single major leak, would stop the music. I suggest you carefully inspect the interior for leaks, especially ones that might have happened suddenly. Also, was the transmission full of oil when you dismantled it? If it lost its oil (either gradually or suddenly), it would lose its drive.
Was the transmission losing performance (i.e., slower than it used to be when climbing hills) before the loss of drive? This would signify wear. If it was still bounding up hills like a mountain goat, and there were no metal particles in the transmission (especially its oil filter) we would seem to be looking for a sudden failure, not a wear-out failure.
The alternative to a sudden hydraulic failure is a sudden mechanical failure. Have you checked for stripped internal splines, keys etc, as well as gears that have lost teeth or shafts/axles that have broken in invisible places? I've seen pictures of mower transmissions that lost just one or two teeth from a driven gear, and the damage was relatively difficult to see, but there was no drive. Also, I notice that the axles drive the mower's hubs with keys. When a hub drive key shears there is likely to be something of a bang, followed by little or no drive, so please report on the condition of the keys and keyways.
