CUBA has made an unprecedented showing in this year's Pan American Games, much to the visible delight of President Fidel Castro.Cuba's poor economy had called into question whether or not the country would be ready to host nearly 20,000 athletes, journalists, and tourists from 39 countries in the Western hemisphere. The games, held every four years, end this weekend after 16 days of competition. At press time, Cuba was outdoing the United States - the traditional powerhouse of the games - in number of gold medals won, though not in overall medal totals. Many of the US's top track and field athletes had stayed away to train for the World Track and Field Championships in Tokyo later this month. Tuesday the US soccer team scored a significant victory over Mexico, 2-1, to win the gold medal. An overflow crowd of 55,000 (complete with 30 aerobic-champion cheerleaders) attended the much-anticipated US-Cuba baseball game last weekend, which the young US team lost to the more seasoned Cubans, 3-2. Both teams play semifinal games today, and are likely to meet in the finals.