There are many performance crankshafts available for mopeds. These can generally take more stress at higher RPMs than a stock crank, which is perfect for kitted engines. Manufacturers of performance cranks include Top Racing, Rito...
Installing a performance crank alone does not increase your speed. A performance crank can however influence the carter flow and the combustion chamber size.
A performance crank with full "cheeks" can improve the carter flow. Sometimes there are holes in the cheeks, filled out with nylon or cork. This is done to reduce the weight of the crank, increasing gas reaction but reducing low end torque. This is mostly done on high-rpm machines, since they don't need a lot of low end torque.
Some performance cranks have a shorter shaft that allows for greater or less displacement. Shorter or longer shafts are also used to fit cylinders that are not meant to go onto a specific engine. Although it does not fix the timing-problems, it does fix the piston coming above or too much below the cylinder.
A common mistake is that a shorter or longer shaft would influence the length of the stroke. This is not true. The stroke is determined by where the shaft is connected to the crank, how far it is off centre. The further off centre, the longer the stroke.
Some performance cranks come "stuffed". This means that between the wristpin (which connects the piston to the crank), and the crank, there is a ring on needle bearings. These bearings provide less resistance between the crank and wristpin, causing less friction and better performance.