Asterix at the olympic games

The athletes of the ancient world assemble in Athens for the Olympic Games. Asterix and the Galus enter too, but they're due for a setback. As an artificial stimulant, magic potion is banned. Can our friends win at the Games without it ? And what's the sepcial ingredient of the other potion, the one in the cauldron in the shed with the door that doesn't close properly ?