If the copied card has {X} in its mana cost, X is 0.

If a card copied by the token had any "when [this permanent] enters" abilities, the token also has those abilities, and they'll trigger when it's created. Similarly, any "as [this permanent] enters" or "[this permanent] enters with" abilities that the token has copied will also work.

The token copies exactly what was printed on the original card and nothing else. It doesn't copy any information about the object the card was before it was put into your graveyard.