getting rights

Hi there, I am currently trying to find out some information about getting the rights to put on a show. The show is a musical theatrical adaptation of a film. It contains an original music score; however large amounts of dialogue and possibly the title are lifted from the original film. The director (and producer) is dead; and the film is American (if that makes any difference). The film would be put on with an extremely low budget - and would seek only performance rights. I am wondering if anyone can advise me as to whether or not gaining rights would be necessary - and if so how I might go about getting them. Any correspondance on this matter would be much appreciated. Best regards, Zoe.