Bummer, sorry to hear of your troubles. The blue smoke would indicate burning oil, so rings/bores, possibly valve guides. The oil light comes on at a very low pressure, so little or no oil feed to the crank, probably due to lack of oil in the tank judging by your description. You may have damaged the big end bearings though by running it in that condition, indicated by a heavy knocking noise when the engine runs. As for the oil leak, difficult to say without information on where it's coming from but odd that this apparently started after the problems. If it were mine I start stripping down from the top until I found the cause (but it sounds like maybe a full rebuild is on the cards).