This has been a tough week for Ty Stofflet, the 41-year-old past and possibly dethroned king of fast-pitch softball. The first batter he faced in the opening game of this National Sports Festival hit one out of the park. Two days later the 25-year-old kid who has usurped his throne outpitched him, shut out his team and killed him with kindness.

"I grew up watching him pitch," said Dave Scott, the right-handed son of a famous Pennsylvania softball pitcher. "Other than my father, he's my idol."

Lately, Stofflet has been hearing himself talked about that way, like a museum piece let out for the weekend. People expect him to take the mound with gold plate on his shoes. But Stofflet says he is a few thousand strikeouts from the end of his 27-year career and not at all ready to concede the king is dead.

"I let my arm do the talking," says the man who has won two national titles and one world championship for his team. In 1976, in a world championship game against New Zealand, he pitched a no-hitter for 20 innings, then won the game himself with a hit. Last year he pitched his 25th perfect game.

Fast-pitch softball has always been a pitcher's game. From just 46 feet away, pitchers can deliver a ball at more than 80 miles an hour and make it do more tricks than a circus pony. A ball that looks as big as a melon in a pitcher's hand crosses the plate the size of a Ping-Pong ball.

The softball competition here has been a promoter's dream. On the men's side, Stofflet and his East team, made up of players from Pennsylvania and the Washington, D.C., area, are rematched with Scott's West team of Illinois All-Stars for the gold medal Wednesday night. Stofflet and his Sunners dominated the fast-pitch game for the second half of the 1970s. Now Scott and his team are on top. They are the defending national champions and have not lost a tournament this year.

In the women's game, where the mound is six feet closer to the plate, Kathy Arendsen has been customarily devastating. The 23-year-old from Connecticut has a 100-8 won-lost record in the last four years. In 1979 she pitched the United States to a gold medal in the Pan-Am Games.

At last year's Sports Festival in Syracuse, N.Y., Arendsen struck out Reggie Jackson three straight times in an exhibition game. But even Arendsen has shown signs of mortality this week. Monday, after four straight victories, her East team lost to an Arizona club representing the West, 2-0. One run scored on a wild pitch, the other on a passed ball.

The softballers are unique among teams at this festival. In other sports, regional athletes have been thrown together in slapdash fashion. The four softball teams in both the men's and women's competition are the same that advanced to last year's national championship tournament. They bring a new dimension to the concept of amateurism: some arrive in their own luxury travel buses.

"I'd say our annual budget just for expenses must be about $100,000," says one of the players on the West team, who during the rest of the year play in jerseys that advertise ADM, a billion dollar agriprocessing company in Decatur, Ill.

Most of the ADM players, including Scott, were recruited by the company from other teams in the Midwest. In return for winning championships, the players get jobs with liberal leave time to pursue their sport. On some teams at this level of play, it is not unheard of for players to receive bonuses in their paychecks for work done miles from the factory.

"You know you can't make any money playing softball; it's an amateur sport," said the coach of the Sunners, Barry Distasio, who has to work to keep a smile from stretching to his ears.

The Sunners were forced to leave seven players home because of a new Sports Festival rule this year that bars athletes who have been professionals in any sport. The team replaced them with four players from a Salisbury, Md., league.

"It was bad for them, but good for us," said Jim Brackin, a Rockville, Md., computer analyst who knocked in the winning run in the Sunners' first game. Brackin played on the U.S. team that won a silver medal in the 1979 Pan-Am Games. He is 33 years old, and like Stofflet, he is beginning to hear questions from friends who wonder when he will give up his high-speed game for something more suitable to a man his age. Say, slow-pitch softball.

"Anybody can play slow pitch; it takes an athlete to play our game," says Brackin. Stofflet is more emphatic: "Slow pitch is for elderly people. You shouldn't be allowed to start playing slow-pitch softball until you are 35 or older."