NEW YORK — The curtain is going up on the second act for several musicals, both Gotham-bound and Broadway-ready.James Lapine says his 1995 musical “Muscle” is getting a second shot, this time at Chicago’s Pegasus Players Theater, where Stephen Sondheim’s first tuner, “Saturday Night,” had its American debut two years ago. “Muscle” was originally conceived as a one-act companion piece to “Passion,” with Sondheim aboard as composer-lyricist. When the creative team expanded their adaptation of “Passion” to a full-evening piece, William Finn and Ellen Fitzhugh took over as composer and lyricist, respectively, on “Muscle,” which workshopped six years ago, never to be seen again. Lapine says he’s hoping for a world premiere in Gotham next season. Meanwhile, another new musical that has waited a long time for a Gotham premiere is finally ready to open. “Time and Again” bows Jan. 30 at the Manhattan Theater Club, five years after its first production at San Diego’s Old Globe. “There are such enormous changes it’s impossible to talk about them,” says book writer Jack Viertel. Pushed to explain, the book writer obliges: “It’s totally reconceived in terms of size and storytelling. There used to be 16 scenes, now there are eight or nine. At the Old Globe, we were trying to tell the whole novel, which has a plot more complicated than ‘Angels in America,’ ” Viertel says of Jack Finney’s romantic bestseller about a man who time-travels to another century. “The show is probably 25 minutes shorter now, and about 35% of the music is new. (Director) Susan Shulman was very instrumental as dramaturg. She, (composer-lyricist) Skip Kennon and I spent many hours talking about the story before we put pen to paper again.” Pen has also been put to paper again at “A Class Act,” the musical about “A Chorus Line” lyricist Ed Kleban that is transferring to Broadway’s Ambassador Theater after a run at MTC’s Stage II. “There are things we probably knew were wrong with the show,” says “Class” producer Marty Bell of Musical Theater Works, where the backstage tuner originated. “Then someone puts it in print and you go, ‘Gotcha.’ The main problem we knew we had to address has to do with storytelling and clarity about what Kleban’s life meant.” Beyond that, Bell says that his book writers, Linda Kline and Lonny Price, who also directs and stars, always felt the second act was better than the first. “We’ve solved that now,” he says. Musically, the Kleban score has been made brassier to fill the larger Broadway house, a song has been added, and maybe one will be dropped. “The show will dance more, too,” says Bell. Choreographer Marguerite Derricks is staging all the musical numbers now, not just two, as she did at MTC. ‘Mack’ attack Finally, a Jerry Herman musical that flopped on Broadway the first time out is getting ready to return. Producer Jon Wilner has reserved April 23 for the opening of “Mack and Mabel.” Wilner says he has been promised a Nederlander Theater. If one doesn’t open in time, Wilner says he has no problem bringing in the revival in late spring/early summer. (A spokesman for “Jane Eyre” denies rumors that the Nederlanders have invoked a stop clause on that show at the Brooks Atkinson Theater.) “Rehearsals begin on March 15 with Jane Krakowski,” Wilner says. “We’re rehearsing in New York Wednesday through Sunday, and she flies back to L.A. to shoot ‘Ally McBeal’ on Monday and Tuesday.” Working in Wilner’s favor is the impending actors/writers strike in Hollywood. In any case, Krakowski should have a longer than usual summer hiatus from “Ally,” even if a shutdown doesn’t materialize on the West Coast: Most TV shows are taping right up to the end of June, stockpiling episodes for airing well into the 2001-2002 season. As for the original “Mack and Mabel,” it ran only 66 performances back in 1974. “There were many flaws,” Herman admits. “It had so many talky scenes, and it wasn’t romantic enough. You never got to see Mack and Mabel happy and romantic together. And Mack was just unsympathetic.” Over the years, Francine Pascal has reworked the book by her late brother, Michael Stewart, to address those problems. Herman has added a song that shows the silent-film duo happy together and also reprised another to show them “one-upsmanshipping each another, which is fun.” Wilner says he’ll have the entire capitalization for the show this week. At $4 million, it’s low for a Broadway musical. Wilner and Herman seem to be thinking “Chicago,” and indeed this incarnation of “Mack and Mabel” originated last fall at Reprise!, the L.A. version of “Encores!” “It is pared down, but only scenery-wise,” says Herman. “We won’t have all those scenery changes.” With such a brief Broadway run, who will miss them?