Rulings

Leitmotif Composer's second ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered or otherwise leaves the stack without resolving.
A spell's mana value is determined only by its mana cost. Ignore any alternative costs, additional costs, cost increases, or cost reductions.
For spells on the stack with {X} in their mana costs, use the value chosen for X to determine the spell's mana value.
The token copy will have Leitmotif Composer's ability and will be able to create copies of itself.
In the unusual case where Leitmotif Composer becomes a copy of something else while its triggered ability is on the stack but before it resolves, the token will enter as a copy of whatever Leitmotif Composer is copying.
The token doesn't copy whether Leitmotif Composer is tapped or untapped, whether it has any counters on it or Auras attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.