MANSFIELD REVISITED. By Joan Aiken. Doubleday. 188 pp. $13.95; ANTIPODES JANE; A Novel of Jane Austen In Australia. By Barbara Ker Wilson. Viking. 330 pp. $16.95.

IT WASN'T JANE AUSTEN who said that imitation is the flattest form of sincerity, but if it were permitted her in heaven to read Mansfield Revisited -- a well-meaning latter-day sequel to her own masterpiece, Mansfield Park -- the thought might well occur to her. Joan Aiken, daughter of the poet Conrad Aiken and popular author in her own right, says that she attempted a sequel to what has been called one of the most profound novels of the 19th century not out of presumption, but out of "love and admiration." She writes, correctly, "No one could presume to make any attempt to fill the gap left by Jane Austen. And I have not done so." This could well serve as the definitive judgment on Mansfield Revisited.

Joan Aiken has certainly fashioned a story which most Janeites will be unable to resist, once begun, if only to satisfy a frivolous curiosity about what might have happened to Edmund and Fanny, Mary and Henry Crawford, Susan and all the rest after Jane Austen left them in such just and orderly disposition at the end of Mansfield Park.

The problem is, can we believe Joan Aiken's conjectures? Does her sequel advance or enlarge that understanding of complex human relationships which is Mansfield Park's special gift to the reader? The answers to both questions must be no, partly because Joan Aiken does not display even half of Jane Austen's unparalleled wit, intelligence and felicity of style (this is not necessarily an impolite remark, merely an obvious one) and partly because of the sheer gratuity of the effort to add something already complete, to embroider upon what is already perfect.

Mansfield Revisited really offers little more than a faint reprise of the main plot of Mansfield Park, with some necessary reshuffling of major characters. Fanny and Edmund are almost immediately removed from center-stage to Antigua, while Fanny's younger sister, Susan, takes her place as the novel's moral heroine. Susan, like Fanny, is vainly courted by Henry Crawford, having already given her heart to her cousin Tom just as Fanny gave hers to Edmund. Like Edmund, Tom takes most of the novel to appreciate his young cousin's merits, being similarly bewitched the rest of the time by the dazzling Mary Crawford. Even odious Aunt Norris blandly reappears in the form of sister Julia. It is not hard to guess quite early on how things will turn out.

Yet the real disappointment of Mansfield Revisited lies not in its lack of originality or suspense, but in the way moral complexity had been drained from the two heartbreakers, Henry and Mary Crawford. In Mansfield Park, these two are at once the most superficially attractive and subtly corrupting of all the characters; it is precisely their ability to charm which makes them so morally dangerous. Here, Mary Crawford has become of all things a sainted invalid and her brother a largely reformed, sober, worthy, country gentleman. Thus, it is merely a matter of faithfulness to the original, rather than a moral imperative, that the Crawfords once again fail to win hearts. In Mansfield Revisited, affairs of the heart do not vibrate, as they always do for Jane Austen, with the significance of some deeper contest of spiritual values.

THE READER might be more profitably amused by Barbara Ker Wilson's flight of fancy, Antipodes Jane, a novel based on the unprovable but fascinating idea that Jane Austen may have visited the Colony of New South Wales, in the company of her aunt and uncle, some time during the year 1803-4. Certainly, this particular tribute to Jane Austen suffers from no want of originality.

Although it scarcely aspires to profundity, Antipodes Jane is a surprisingly well-written and engaging book, which succeeds not only in drawing sprightly portraits from life of various members of the Austen family and certain distinguished colonial settlers, but also in suggesting a vivid contrast between two worlds: Bath, "that far-off land of pavement and palaces, shops and sedan chairs, Assembly Rooms and Abbey, and perpetual grey skies" and the exotic little garrison- town of Sydney, with its "chain-gangs, gallows, rum and prostitutes" and its outlandish flora and fauna. Despite some far-fetched wrinkles in the plotting, it is diverting to enter into the imaginative exercise of seeing this raw convict settlement from the cool and civilized viewpoint of one such as Jane Austen.

Neither of these books comes close to capturing the elusive spirit of the great writer they wish to honor, but each in its own way may provide a few pleasant hours of respite for readers sated with modernity. By Elizabeth Ward; Elizabeth Ward, born in Australia, is a Washington writer and the author of "David Jones: Mythmaker."