Paul Simon and Art Garfunkel: The Concert in Central Park.'' Warner Bros. 2 BSK 3654 - Simon and Garfunkel fans looking for a nonstop ticket to the 1960s may be mildly disappointed by their recently released live album, recorded in September 1981. There are constant reminders on the LP that this is the 1980s, and that instead of an intimate studio, we're in Central Park, which is filled with a boisterous and noisy crowd of several hundred thousand. To top that, there's a backup band giving the team a more extravagent sound than would have been expected in S & G's heyday. But then, it's easy enough to travel back to the '60s on the grooves of one their old albums. Despite the possible drawbacks of the live setting, Simon and Garfunkel perform very well, and do manage to revive much of the old magic in tunes like ''Scarborough Fair'' and ''The Boxer.'' They also perform some of the songs they have done since their split. A snappy version of the Everly Brothers, ''Wake Up Little Susie'' - which has made its way onto the charts - is a bonus. Not everything is perfect, certainly - a rather ragged ''Mrs. Robinson'' will prove that - but in a recording from a live performance a few weak moments are understandable. Art Garfunkel can perhaps be forgiven for bragging during the performance that ''I thought it would be somewhat crowded (in Central Park), but we seem to have filled the place.''