The rope trick
The rope trick is a technique you can employ to prevent your crank from turning. It is effectively an economical replacement for a piston stop.
To perform the rope trick, unscrew the spark plug from the spark plug hole, and begin feeding a long piece of rope or something similar into the cylinder. The idea is to fill the cylinder with enough rope that the piston is not able to reach TDC (Top Dead Center), at which point it begins moving back down towards the crankcase. Once you have stopped the piston from moving, you can begin working on components attached to the crank.
When performing the rope trick, take care to avoid getting a piece of rope (particularly frayed ends) from getting caught under a ring or in a port. One of the best ways to prevent this from happening is to make sure that the top of the piston is above the exhaust port before feeding the rope into the spark plug hole.
Check out Moped TV (click the on demand button to browse old episodes) to see a video of using the rope trick to stop the movement of the piston.