Silambarsan is not a part of Ko anymore. The actor was initially signed up by director KV Anand but the recent buzz is that he opted out of the film. According to the actor, the role required him to change his appearance drastically, which meant he could not concentrate on other films.
There are also rumors that Jeeva has been signed up now to star in place of Silambarsan. The music for Ko has been scored by Harris Jayaraj.