If you want to perform maintenance tasks to your TGB 303R 125 then it is usually a good plan to have access to a Workshop Manual for a reference just in case you need to know how to strip off a certain part much like the Exhaust and fit it again accurately.
TGB 303R 125 Workshop Manuals gives you precise diagnostic guides for the vehicle so if something is wrong it will help you work out what the issue is and how you can correct it. Then you are able to assess if you can do this on your own or if ever the job is too big it is possible to make a reservation for your 303R 125 into a decent car garage.
TGB 303R 125 manual