You want to beat Kareem Abdul-Jabbar for 20 points and an NBA championship? Hand off to Herschel Walker? Beat Craig Stadler on the last hole of the Kemper Open?

See Nat Allbright. Then close your eyes.

"Malone, bounce pass to Gary Wise," says Allbright, raising his voice, which is a compelling cross between Mel Allen and Sears' best chain saw, over the sound of a capacity crowd at the Spectrum. It's the Lakers against the 76ers for the NBA title, and mild mayhem pulses out of the cassette deck. "Wise, right side, sets, shoots, misses! He grabs his own rebound, goes up, pumps--scores!" The crowd goes wild. "Fourteen points for Gary Wise, the Sixers' free agent," blats Allbright.

Gary Wise, "the Sixers' free agent," is actually a 14-year-old from Potomac whose father found him a novel birthday present a few weeks ago: personalized play-by-play by Nat the Cat.

Nat Allbright is 55, a veteran radio sportscaster, proprietor of a small advertising agency, owner of a large Arlington townhouse and a far larger imagination.

On the floor of the empty recreation room he hopes to turn into a recording studio soon, Allbright opens a battered black briefcase stuffed with randomly folded paper scraps, letters and audio tape cartridges.

"Here y'got your major league stadium," he says, as he takes more tapes from a plastic K-Mart shopping bag and stacks them on the floor. He holds up one labeled "Continuous."

"Right here: 70,000 fans," he says. He keeps them in the hall closet when he's not using them, which is not very often, lately.

There are other tapes, some on endless loops that play continuously and are faded in and out by Allbright at appropriate moments: the crack of bat against baseball, a crowd chanting "Go! Go! Go!", the national anthem, country-club applause, signal-calling at the line of scrimmage, marching bands, horses rounding the far bend, even real commercials for beer and cigarettes.

Allbright is no stranger to sports fantasy. "I started recreating baseball games, from box scores I'd cut out of the Roanoke Times, when I was 8, maybe 9 years old," he says. "People used to come in my room and say, 'Nat, what's the score?' "

For 12 years, until 1961, he recreated Brooklyn Dodgers games for a 115-station radio network that reached from Cleveland to Miami, working with his voice, a few crowd-sound effects and inning-by-inning reports brought into the studio by a runner.

During the baseball strike in 1980, he made national news when he created an All-Star game and broadcast it over WEAM-1390, the Falls Church big-band station where his daily sportscasts normally cover events that actually occur.

About a year ago, just before he made a few more headlines by broadcasting eight Redskins "games" canceled by the players' strike, Allbright had an idea: Why not take requests?

"I was at the bowling lanes," he says, "and this fella and I were talking about golf, and I said to him, out of the clear blue, 'How would you like a tape of you playing the big guys--Nicklaus and Palmer?' He said, 'Sure,' and that was it."

So now, while Jim Bouton has a company that will put your picture and playing stats on a baseball card, Nat Allbright runs a one-man business that puts you in the game, any game, with the outcome or ending of either your choice or Allbright's imagination. It's 25 minutes of ersatz glory, cheers included. He charges about $25, and says the requests have been running four or five a week.

"It's the greatest thing since . . . the advent of time," Allbright says, in characteristic understatement. "No, really, what it is, is an ego trip. A chance for a regular guy to hear his name right in there with the big names. People can say they played in the same outfield as Mickey Mantle, or won a game on a hit off of Bob Gibson, or Jim Palmer. Or they can get in the ring with Muhammad Ali. I had one guy wanted to get knocked out by Sugar Ray Leonard."

One of Allbright's customers once played a tape, for an unsuspecting business associate, in which the customer, who is 6 feet 2, 245 pounds, played on the Dallas defensive line alongside Randy White, Harvey Martin and Too Tall Jones in a big "game" at RFK.

The friend listened quietly for a while, then said, "I didn't know you played pro ball. You know, this is incredible--I saw this game."

Yeah, and so did Nat Allbright.