Since you completely own the car from the beginning when financing it with a car credit in Switzerland, you are free to do with your car what you want.
Whether it is modifications to accommodate your hobbies, your stylistic preferences, or performance, you are allowed to make any changes to suit your purposes.
However, keep in mind that any modification must adhere to Swiss road and traffic regulations defined by the federal traffic administration ASTRA and needs to pass the motor vehicle inspection.