By William Ferguson It would as far too cynical to tell an actress only two years into her career that she was born too late.  But Moira Kelly is as close to Maureen O'Hara as the 1990's is going to get- a nice Irish-Catholic girl who, yes, will do a nude orgy scene with midgets in a David Lynch film, but won't like it a bit.  And not without consulting a priest first,  Really. "Nudity...I have to say I will never do it again, Kelly says with unstudied earnestness.  "I hope I hit that next level really fast so I can make the choices and decide what is right and decent for me to do." What has so far been right and decent are memorable roles in forgettable movies. (Remember her keening frailty as a teenage murderess in Love, Lies, and Murder? No?) But now Kelly has a chance to be seen- fully dressed and with gray hair - in Charlie, director Richard Attenborough's take on Charles Chaplin.  As Oona Chaplin, Kelly leaves the realm of startled-fawn adolescence to play a character who ages nearly three decades onscreen.  "Oona was hard," she says, "I had to age her to forty-six and that's not really physical aging, it's more of an internal maturity." The buzz on Charlie all but insists Kelly will finally shake the Michael Caine syndrome (good performance, lousy film). But that should have happened with Billy Bathgate (she played Dustin Hoffman's girlfriend, remember?)  and the Twin Peaks movie.  If Kelly is cynical about anything, it's imminent stardom. "First of all-I only believe what I see.  If, perchance, it's true, and people are really flung back by it, I just hope by then I'm strong enough to handle what comes next." She may sound stuffy (perchance?) and unreasonably moral for a twenty-four-year-old actress.  But that's Moira Kelly's charm - her, dare we say, inner maturity.