If you wish to carry out adjustments to your MV Agusta 125 S then it is always recommended to have a Workshop Manual for reference if you need to know methods to remove a particular component for instance the Gearbox and fit it again accurately.
MV Agusta 125 S Workshop Manuals will provide you with precise diagnostic details on your vehicle so if something is playing up it can help you understand what the issue is and ways to resolve it. You will be able to determine if this can be achieved by yourself or in the event the job is simply too big you can actually make a booking for your 125 S in to a suitable mechanic.
MV Agusta 125 S manual