If the copied artifact is a token, the new token that's created copies the original characteristics of that token as stated by the effect that created the token.

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

Spells can only be cast for their sneak costs any time you could play an instant during the declare blockers step on your turn (after your opponent has decided whether to block).

If a creature spell's sneak cost was paid, the creature it becomes enters tapped and attacking the same player, planeswalker, or battle as the creature that was returned to its owner's hand to pay its sneak cost. This is a rule specific to sneak; in other cases, when a creature is put onto the battlefield attacking, that creature's controller chooses which player, planeswalker, or battle it's attacking.

The token copies exactly what was printed on the original artifact (unless that artifact is copying something else or is a token; see below). It doesn't copy whether that artifact is tapped or untapped, whether it has any counters on it, or any Auras or Equipment attached to it, or any non-copy effects that have changed its types, color, power and toughness, and so on.

Although a creature put onto the battlefield this way is attacking, it was never declared as an attacking creature. Abilities that trigger whenever a creature attacks won't trigger when that creature enters attacking.

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

If the copied artifact is copying something else, then the token enters as whatever that artifact copied.