My bet would be a piece of piston skirt fell off and got below conrod. Punching an hole in the alloy sump is one thing, punching an hole through the steel stiffener is a bit harder. Seems to have done both. Dropped a valve or valve contacted piston rocked piston and cracked skirt, in use it fell off. Result hole in sump. Who changed the chains? Damage could have been done to piston turning engine with starter after replacing camshaft if tappets had expanded and not been allowed time to relax before turning engine over. When doing work of this kind always allow time for tappets to relax and turn engine by hand at least two revolutions to check for fouls.