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

Measurements:

Wire Diameter: 4.0mm
Inner DIameter:44.0mm
Length: 108.0mm