but did you confirm smooth operation of the oil pump itself?
its not uncommon after an engine failure for aluminum transfer to the pump gears and they dont turn to pump and just spin as a complete lump not pumping any oil
i see no way to avoid going back in at this point because you need to stop guessing and find out for sure whats knocking
next time buy a used mower with a bad deck and good running motor of the correct or larger hp size for $100-$150 and be done with it
repairing an engine that had a major failure is never a good idea because very often things like this happen after you have put good money into them