Opera rewrites: Gerard Schwarz gives tampered versions a staging.

History is littered with operas that faced, and succumbed to, total oblivion: Either they were misunderstood, or their popularity had waned for a newer generation who wanted something different.

The masterworks usually found a way to survive, but more often than not because some later composer devoted his specific attention to one of them. The most famous example of this attention was Rimski-Korsakov's reworking of Mussorgsky's ''Boris Godunov.'' Without Rimski, it is safe to say that ''Boris'' would have been dismissed as the unperformable rantings of a demented genius. With Rimsky, the work quickly became a permanent fixture in the standard repertory.

Later Shostakovich got on the bandwagon, and he gave the contemporary world its first real taste of what the entire Mussorgsky original might sound like. Today, we have that original to the exclusion of the Rimski, and even the Shostakovich, both dismissed as tamperers. Yet both those versions of the opera create a viable, meaningful, artistic encounter and should be heard in addition to the original.

Recently, Gerard Schwarz - music director of both the Mostly Mozart Festival in New York City and the Waterloo Music Festival in Waterloo, N.J. - offered the chance to hear two operas as reworked and rewritten by famous composers: Wagner's edition of Gluck's ''Iphegenia in Aulis,'' and Richard Strauss's overhaul of Mozart's ''Idomeneo.''

It is hard to imagine, now that ''Idomeneo'' has been recorded four times and has gained an important foothold in the international repertoire, that not too long ago it was an unknown commodity for most people. Richard Strauss adored Mozart. He was constantly referring to his own operas as Mozartean, begging conductors to approach his music as they approached Mozart. He even used Mozartean dramatic situations in his own operas.

No wonder, then, that he would want to take Mozart's first important opera seria - and one of his greatest works - and make it accessible to a reluctant public. What better way, he thought, than to make it palatable to his particular audience, trim the length, reorder the musical numbers, add a few interludes and scenes to better explain the action, and throw in some vintage Strauss harmonies to liven things up musically.

In those days, it was still considered perfectly permissible to alter a composer's manuscript. Conductors were forever ''improving'' Beethoven and Schumann, adding notes and harmonies even to then-new composers like Sibelius. Now we find that exercise of will repellent, yet in some cases that will has created a hybrid version of a piece that is in its own right a marvelous musical experience.

Such is the case with Strauss's ''Idomeneo.'' When Gerard Schwarz programs this sort of work at the Mostly Mozart Festival in New York, he is enriching his festival and doing a great service to music lovers. The performance last Friday convinced me that the Strauss version ought to be presented on stage. It is not really Mozart, but it is unique. Unexpectedly, Strauss interjects his own special brand of horn solo, a self-quotation, or an ensemble that rivals the best of ''Rosenkavalier'' or ''Intermezzo,'' particularly the dazzling quartet that precedes ''Idomeneo's'' final chorus.

The performance, sung in Strauss's German, gave us the flavor of the work; the singing gave us young artists in the process of becoming important ones. Jerry Hadley, who sang the title role, is well on the way to an important career. In this role, which he has done in Europe, he has as yet to learn about depth of characterization, but he handles the role and himself well. Alessandra Marc sings the role of Ismene - Electra in the original Mozart - with ease and facility in the floridity. She sings everything in a plangent style that becomes a bit tiresome, but the sheer size of the voice and the richness in the middle and upper reaches (she has not even begun to explore her low register) are the mark of someone uncommonly rich in potential. Veteran John Macurdy also impressed this critic in several small roles.

Mr. Schwarz was responsible for another exhumation of sorts, the Wagner/Gluck at the Waterloo Music Festival. The work is far less interesting than the Strauss/Mozart. Wagner has tended to make ''his'' ''Iphegenia'' monochrome, low on vocal virtuosics, and ultimately not very communicative, though clearly his intention was to make it speak with simple eloquence. Miss Marc was cast in a completely compatible role as the long-suffering Iphegenia, and the way she used her richly colored instrument to create a musical mood was consistently impressive. Justino Diaz was mightily impressive as the high priest Arkas. Otherwise, the singing ranged from the acceptable to the less than acceptable, despite the presence of such names as Norman Bailey (a gruff Agamemnon) and Katherine Ciesinski (a shrill Clytemnestra). Mr. Schwarz seemed less inspired by Wagner/Gluck than by Strauss/Mozart, but the Waterloo forces performed commendably.