I wouldn't describe Riff as "the lead role" in West Side Story. Tony and Maria are the leads. Riff is in with Bernardo and Anita as the first tier of supporting roles. (Chakiris and Moreno won the Supporting Actor and Suportting Actress Oscars for Bernardo and Anita.) I mean, nobody describes Mercutio as "the lead role" in Romeo and Juliet.Russ Tamblyn was the lead role in West Side Story, and he was very good in that.
Well, maybe the singing part isn't necessary. Natalie Wood didn't sing Maria's songs (or at least it isn't her voice that made into the final soundtrack of the movie).