IF EVER a superb 19th century play were ripe for musical adaptation on the American stage, Cyrano de Bergerac is it. The swashbuckling hero, the ever-so-radiantly-beautiful heroine, the villain, the grand gestures, the love poems and pathetic deaths-any playwright who missed out on Man of La Mancha would have to be almost a genius to blow theis opportunity.

Anthony Burgess is undoubtedly a genius. If it weren't for Christopher Plummer's nearly flawless performance and the percise, well-conceived staging of director Michael Kidd, Burgess might have succeeded in turning Edmond Rostand's intelligent play into the sentimental pap Cyrano de Bergerac clearly wasn't. Cyrano the new musical running three weeks in Boston before its Broadway debut, muffles the impact of the original play.

Burgess did have the sense to stick to Rostand's structure. Cyrano de Bergerac, France's greatest swordsman and a distinguished poet, is hated by the nobility for his iconoclastic boorishness and unflinching sense of independence. Admired by friends, loathed by enemies, he is cursed by his grotesquely protuberant nose. Because of his ugliness, he cannot confess his deepest secret-a passionate love for his cousin Roxana. But when the heroine falls in love with an Adonean but doltish young soldier, Cyrano offers to help him by writing the love-letters whose beauty win Roxana's heart. Christian, the beautiful youth, seeks triumph of the flesh; Cyrano, the poet, seeks glorification of the spirit.

Two factors make Cyrano de Bergerac a difficult story to stage in any form substantially different from the original. Rostand created an extraordinarily subtle character in Cyrano. The dominant theme of his behavior is a vain defense against feeling ugly-his violence, his grandness, his sense that all the world is his enemy. Roxana eventually discovers that she has been in love with Cyrano's soul, but Cyrano dies a moment afterwards, having been ungraciously clobbered with a log dropped from a window.

Still, Cyrano's motto- "I have decided to excel in everything"-is more than mere bombast. His poetry is a celebration of the spirit. He is the enemy of cowardice, weakness, and stupidity. His white plume flies unsullied to the romantic-tragic end, although you must ask if it was worth his self sacrifice and emotional blindness.

The second problem with adaptation is that the values of the play are not timeless. The sense of glory the play exudes depends on the director's faithful recreation of the romantic atmosphere of an earlier century. Love songs which accentuate the martial metaphors for love and sentimental ditties which undercut Cyrano's depth of feeling erode the play's force as a triumph of the heart.

CONSEQUENTLY, Cyrano is most entertaining when it most closely approximates Rostand's play's. Plummer, as Cyrano, is throughly moving in the play's balcony scene, wooing Roxana from the darkness while she thinks he is young Christian. The other players fall far short of Plummer's commanding performance. Leigh Beery, as Roxana, is a better singer than an actress. In his role as Christian. Mark Lamos projects little personality. Some of the finest performances come from supporting players-- Arnold Soboloff as the poet Ragueneau, and James Blendick as Le Bret, the captain of Cyrano's company.

Michael J. Lewis's music is usually pleasant. only occasionally rousing. Burgess's lyrics are intermittently clever, but advance the spirit of the play only rarely. One song, "From Now 'Til ael should be dropped from the score.

A BETTER ADAPTATION might have relieved the play of some of its cumbersome artifices, especially the falling leaves in the final scene: the death bell peals, the leaves fall, Cyrano enters for the last time-get it?

But the musical maintains a satisfactory pace because of Kidd's eye for stage movement and because Christopher Plummer does Rostand's lines so well. It would be far, far better to see Plummer in the original play, but as Cyrano's opponents discover, in swordfights or musicals, you take what you're stuck with.