Now that is not an easy task but it's very important because music is essential to setting the mood to scenes. You have the entire gamut of musical compositions to choose from. Stanley Kubrick understood this if you remember the musical scores from "2001" and "Barry Lyndon," to mention only two of his films. You might consider "Mars" from "The Planets," or one of the songs from "Carmina Burana," which have been adapted for video games as well as movies. And don't forget "Ride of the Valkyries" from Act III of "Die Walkure," and how that was perfectly adopted for the helicopter assault scene in "Apocalypse Now." Made everybody's hair stand on end. Not that you'd select any of those for your script, but they are only mentioned as examples others have used to good effect. Myself, I'd set the entire movie to "Garry Owen." Get Leonard Bernstein to write the score, except he's dead.