Asterix at the Olympic Games (Asterix #12)

The athletes of the ancient world assemble in Athens for the Olympic Games. Asterix and the Gauls 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 special ingredient of the other potion, the one in the cauldron in the shed with the door that doesn't close properly?