To burlesque an opera, you'd better have flair

Non Pasquale. Pop opera based on ''Don Pasquale,'' by Gaetano Donizetti and Giovanni Ruffini. Music adapted by William Elliott from original arrangements and orchestrations by Tito Schipa Jr. and Gianni Marchetti. Additional orchestrations and music coordination by Roy Moore. New libretto by Nancy Heiken and Anthony Giles. Directed by Wilford Leach. Choreography by Margo Sappington.

When director-designer Wilford Leach and conductor-arranger William Elliott mounted their wild and crazy version of ''The Pirates of Penzance,'' they had their fun with W.S. Gilbert's libretto. But they were scrupulous about preserving the rich melodiousness of Arthur Sullivan's lovely score.

Such is not the case with the updated ''pop opera'' version of Donizetti's ''Don Pasquale,'' which is closing the 1983 season of free performances at the Delacorte Theater in Central Park.

Apart from adding farcical embellishments, a clutch of adapters and arrangers (see credits above) has remained reasonably faithful to the Donizetti-Ruffini libretto. But the perpetrators of this curious monstrosity have played hob with the score. As a result, much of its melodic beauty has been drowned in the metrical spaghetti sauce.

To demonstrate the pervasive ''non'' in ''Non Pasquale,'' the rowdy action is preceded by a prologue in which a character named Trumpet (Joe Grifasi) derides operatic tradition and explains more or less what is about to happen. Mr. Grifasi, who serves as combined chorus and second banana recalls the comedianship (but not the skills) of old-time dialect specialists like Henry Armetta. In the present proceedings, however, Trumpet should be muted.

The pop opera at the Delacorte was inspired (if that is the word) by the Donizetti-Ruffini farce. It tells how Pasquale (Ron Leibman) schemes to marry a young beauty (Priscilla Lopez), thereby simultaneously disinheriting his nephew Ernesto (Kipp Tozzi). Fortunately for Ernesto, his friend, Dr. Malatesta (Joe Masiell), arranges a masquerade of mistaken identities with which to gull the foolish old man.

After all the shenanigans comes the requisite happy ending.

Although performed by a company of attractive and talented singing actors and dancing singers, ''Non Pasquale'' has nevertheless gone badly awry. It mistakes opera buffa (farcical opera) for burlesque. The good sense and good judgment that guided Messrs. Leach and Elliott in the case of ''The Pirates'' has forsaken them in the present shambles. It seems doubtful whether Donizetti would recognize his own score. A comic masterpiece has been reduced to a travesty.

In fairness, it should be reported that what seemed to me an interminable evening gave evident pleasure to many who attended the press opening of ''Non Pasquale.'' However, the progress of the performance was not without its defectors in the audience. (Perhaps they had trains to catch.)

Considering Broadway's present top-ticket price of $45 for musicals, the prospects that ''Non Pasquale'' might join such New York Shakespeare Festival transfers as ''A Chorus Line'' and ''Pirates'' would seem fairly dim.

Meanwhile, the pop beat goes on at the Delacorte. Mr. Elliott, the dynamo of the orchestra pit, conducts with his customary unflagging atheleticism and energy. Aided by the usual amplification, the 13-piece orchestra (dispersed in three sections) fills the night air with sound and fury and rhythmic distortions Donizetti never dreamed of.

Speaking of the night air, I should not forget to add that Delacorte performances are often enhanced by mother nature's amenities. ''Non Pasquale'' was no exception. As the riotous second act progressed, a nearly full moon rose majestically and unconcernedly across the Manhattan sky. It was a sight to remember from a night to forget.