Neil Simon's quirky, funny new comedy recalls his youth; Brighton Beach Memoirs Comedy by Neil Simon. Directed by Gene Saks.

The autobiographical hero of Neil Simon's new comedy fantasizes his baseball-diamond triumphs as a star pitcher for the New York Yankees. Instead, Neil/Eugene became not only a writer but a home-run champion of stage, screen, and television.

As a playwright, Mr. Simon has drawn widely from his personal history - notably in such stage fare as ''Barefoot in the Park'' and ''Chapter Two.'' But ''Brighton Beach Memoirs,'' at the Alvin Theatre, is his most autobiographical work to date. The affectionate and sometimes rueful backward glance recalls a September week in 1937 when the crowded little dwelling of a Jewish-American family becomes suddenly engulfed in crises.

The novelty company for which Jack Jerome (Peter Michael Goetz), the head of the household, has been moonlighting as a salesman goes out of business. Jack's widowed sister-in-law, Blanche (Joyce Van Patten), must make decisions for which a sheltered life has not prepared her. Jack's normally reliable elder son Stanley (Zeliko Ivanek) loses his $17 weekly wages at poker. Blanche's elder daughter, Nora (Jodi Thelen), is told that she cannot quit high school to audition for the chorus of a Broadway musical, entitled ''Abracadabra.''

All this and more is filtered through the eyes and sensibilities of the callow, teen-age Gene, an alternately perplexed and perceptive observer with composition book always at the ready. Acted by Matthew Broderick with a marvelous combination of unselfconscious precocity and downright amazement, Gene is the play's narrator and commentator, a one-man Greek chorus in brown knickers and scuffed sneakers. As both guide and protagonist, he furnishes the ideal liaison between the 1983 audience and the 1937 world of Brighton Beach.

Mr. Simon has packed it with incident and occasion. There are the family meals including a dinner of liver and cabbage (''the ultimate tragedy,'' says Gene). There is the bitter but heretofore unexpressed sibling hostility between Blanche and Jack's wife, Kate (Elizabeth Franz), that erupts in the second act. There are the scenes in which the more worldly Stanley contributes luridly to Gene's sex education. Finally, there is the distant Nazi menace, brought home to Brighton Beach with the news that some Polish family members who have escaped Hitler will soon be arriving in America in need of shelter.

As the foregoing perhaps indicates, ''Brighton Beach Memoirs'' is not the tidiest of plays. It moves with leisurely pace as Mr. Simon accumulates the details of each personal dilemma and painstakingly works his way toward a plausible resolution. Although it seems at times pitched to an unduly strident emotional high, the performance staged by Gene Saks is dedicated to the concern that informs the credo of the scolding Kate - ''I don't want anyone in my family to get hurt.'' And that includes Blanche's pampered younger daughter, Laurie (Mandy Ingber).

In any case, as Gene observes presciently at the outset, ''This is going to be one heck of a day.'' Mr. Simon proves his bright alter ego abundantly right in this odd, funny, and quirky play. The cozily shabby Brighton Beach setting was designed by David Mitchell, with costumes by Patricia Zipprodt and lighting by Tharon Musser. Recorded snatches of the big-band sounds and popular radio signatures of the 1930s help set the mood and recapture the atmosphere of a bygone era.