Walk Behind Lawn Mowers

I have an old walk-behind mower, the Royal 53S. The self-propel is not working and I figure I need to change the belt. Does any one know how to remove the belt on that model? It's possible the belt is okay and just the tension pulley needs a new spring, but given the belt is 20 years old, I'm figuring that is the problem. Any other suggestions are welcome.