Compression springs do a couple of things. First they supply the pressure to engage the belt. They also determine the rate at which the CVT transmission changes from low to high gear. A 1000rpm spring will go into a higher gear sooner than a 2000rpm spring.
This NCY 1000 rpm compression spring will fit:
Yamaha Zuma 2t 50cc
Most Minarelli applications
Wire Diameter: 4.0mm