Below is a list of good persuasive speech topics related to sports (courtesy of Buzzle.com):
Greatest NBA players of all time.
Steroids and sports.
Is World Wrestling Entertainment real.
Meditations help sportsmen.
Karate or Martial arts be made compulsory in schools.
Countries should focus on promoting women's sports.
Extreme sports are not meant for all.
Are fantasy sports as good as real?
Which is the best: Badminton or Tennis?
Can there be a single greatest sport person?
Sports person's salaries should be based on performance.
Should shot put and javelin throw be given equal importance as other sports?
Is match fixing the cause of declining popularity of some sports like cricket?
Should hunting be considered as a sport?
For more ideas, check out Bleacher Report's Top 30 Arguments and Debates in Sports.
And here are 6 Tips for Writing a Persuasive Speech (On Any Topic), featured at TIME.com.