Just had a brief look on the 2010 125cc, it migth work, however the distance from the pivot bolt to the pushrod mount is 10mm shorter on the 2010 125 compared to the 2012 300. 50 mm vs 40 mm. I dont know if the older 300 used the same brake pedal as the 125. There is also a bit of pivot bracket offset difference, the 2010 sits deeper.