Pivot bolt sizes are different. 15 vs 12mm. And the 125 arm has thinner axle mount plates so chain adjusters are different. Pivot widths most likely are slightly different. Sure you could probably make one fit with some work converting down to 12mm pivot size, narrowing pivot if required etc etc but i don't see the point. You should be able to find the correct 125 arm which is has all the correct dimensions to fit with the 125 frame and wheel and be straight bolt on. I see them often for sale on ebay, usually advertsied as TT500 or HL500 swingarms
