If you want to do fixes on your own Derbi xRace 50 SM then it is always a good plan to have access to a Workshop Manual to use as reference if you need to know easy methods to strip off a certain part much like the Brake Discs and match it again perfectly.
Derbi xRace 50 SM Workshop Manuals will provide you with specified diagnostic wisdom for your automobile so if something is faulty it can help you understand what the problem is and tips to correct it. After that you can analyse if you can do this by yourself or if the job is too big you could book your xRace 50 SM into a nearby mechanic.
Derbi xRace 50 SM manual