It's unusual for old movies to get a brand new soundtrack, years after their release. The only example I can think of offhand is Metropolis, which got Freddy Mercury and Giorgio Moroder tunes. But over at The Stranger, there are some fascinating suggestions for present-day artists who should score classic SF films.
Among other things, they want to see Boards of Canada do a new score for the 1951 Day the Earth Stood Still, and Black Moth Super Rainbow do a new soundtrack for Zardoz. If you could pick any classic science fiction movie and assign any current recording artist to do a soundtrack for it, who would you pick? And why?