If the spell that's copied has an X whose value was determined as it was cast, the copy will have the same value of X.

The copy will have the same target unless you choose new ones. If you change the target, the new target must be legal.

You can't choose to pay any additional costs for the copy. However, effects based on any additional costs that were paid for the original spell are copied as though those same costs were paid for the copy too.

The copy is created on the stack, so it's not "cast." Abilities that trigger when a player casts a spell (such as conspire's triggered ability that creates a copy of the spell) won't trigger.

Any damage dealt to creatures by a source with wither, whether it's combat damage or noncombat damage, is dealt in the form of -1/-1 counters.

If the spell that's copied is modal (that is, it says "Choose one —" or the like), the copy will have the same mode or modes. You can't choose different ones.