JAMIE ASTAPHAN, the doctor at the center of Ben Johnson's steroid scandal, said the disgraced Olympic sprinter owes him $30,000 for past medical treatment, and he is willing to go to court to get it. ''If it means going to court, I will take him to court. . Ben won't pay me,'' Astaphan told the Toronto Sun from his home on the Caribbean island of St. Kitts. Johnson, who lost his gold medal and world record in the 100-meter sprint after testing positive for anabolic steroids at the Seoul Olympics, testified at a Canadian inquiry he was on a steroid program set up by Astaphan and former coach Charlie Francis. Astaphan said it would not be a bad idea if he and Dr. Mauro di Pasquale, a Canadian steroid expert, were put in charge of drug testing for the 1992 Olympics in Barcelona. ''Nobody could fool either one of us,'' Astaphan said.