4200 Changed oil,flushed radiator,filled antifreeze ,tractor would not start, finally starts but alot of smoke,doesn't sound right,not over heating but knocking,let it run ,get big bang and piece of metal flies off,oil leaking
That's not good.
Knocking, a big bang, engine parts flying off and then oil leaking sounds like a catastrophic failure (threw a rod?).
From here at my desk, it sounds like an engine rebuild or new engine may be in order.