I just read about this bike over breakfast (morning bowl of gruel). The YZ125 MX version has not changed in years. For the 125X Yamaha added the 18 rear wheel and kickstand. And the tank is the same, small for longer trail rides though. Then, If I remember the internal changes correctly, there is a change in combustion, and the power valve, and how it is timed. I believe there was a change in the setup of the shock and fork but do not remember what. Since this is a newer bike I am not certain if the actual shop manual is available yet. I would guess, and do not quote me on this, but for most of the parts and maintenance the shop manual for the MX would help.
Mully