There's no quicker way to increase your Radio Controlled Car performance than a motor upgrade. When choosing an upgrade motor ensure that the electronic speed controller which you intend to use is capable of running the motor, most speed controllers will quote a motor limit, this is because the quicker the motor, the greater the current drain. Fitting a high performance motor without conforming to the limits of the ESC can make the speed controller run hot, sometimes causing it to overheat and possibly fail (the standard speed controller in a Tamiya kit or Ansmann car will handle a motor down to about 21 turns). It may also be worth considering efficency of your car with a Ball Race Bearing Set and heavier duty connectors such as the deans type

It is also important to note that the quicker the motor the less runtime you will get, for example a standard 540 motor can run for about 10 minutes whereas a 21 turn motor wiil run for about half that time. So upgrading to a higher capacity battery will help to maintain your runtime. Brushed type motors, unlike there brushless equivilents require regular maintanece. It is important to make sure that they are clean of any debris inside the motor and to use a cotton wool bud coated with motor spray to clean the commutator of the motor of any carbon debris from the brushes and also to replace these motor brushes when they have worn down to half there original length.