One of the best musical-comedy films, Singin' in the Rain, was Gene Kelly's baby.     To stage it has been a long-cherished dream for Australian showman David Atkins.     But the apparent commitment, effort and money poured into the stage show are not enough to save it from being flat.     Most musicals make everything look easy, but Singin' seems too laboured.     On the big Regent Theatre stage, the cast seems tired and depleted while working extra hard to gain momentum.     The punchy pace that is so crucial to musicals is interrupted by several film sequences that make the show much longer than needed.     The first act seems particularly drawn out.  The second act is scatty.     There are, however, some magnificent splashes -- the splendour of Singin' in the Rain and the simmering restraint of Broadway Melody.     The story followed two silent-screen idols, Don Lockwood (Todd McKenney) and Lina Lamont (Jackie Love), as they make the transition to talkies.  Lockwood's long-time friend Cosmo Brown (Wayne Scott Kermond) helps in the transition and in encouraging the romance between Lockwood and up-and-coming actor Kathy Seldon (Rachael Beck), to Lamont's consternation.     McKenney makes the Gene Kelly role his as a less assertive, but more exciteable, Lockwood.     McKenney's dancing is graceful and exuberant.     Beck is a sprightly and finely voiced Kathy and, while he may not have the comic charm of Danny Kaye (Transcriber's note: er ... does she mean Donald O'Connor??) in the film, Scott Kermond is just as agile and athletic.  Sheree da Costa, as the Cyd Charisse soloist, is delightfully restrained and glamourous.     Love has perfected Lamont's squeal to be a little too annoying.     A very committed cast in a show that only has flashes of brilliance in a rainy sky.