Hello.
Here is some measurements fro an 86 KX500 Shock.
Obviously the spring info can change depending on adjustment and rates, ect:
All dimensions are approximate:
Spring thickness: 12MM OD -
Spring Diameter: 88MM -
Spring length as mounted: 267MM -
Upper mount tab thickness: 32MM -
Upper mount tab hole ID: 12MM -
Lower Mount Internal Width: 26.5MM -
Lower mount hole ID: 10MM -
Eye to Eye: 420MM
Before you buy a 125 shock, simply compare the measurements.
Also, a 125 shock will almost certainly have different internal dampening, rebound, and spring size.