|MadSci Network: Engineering|
Increasing the strength of the magnet will change the performance of the motor. Unfortunately, this comes at a price. One can get rare earth magnets that have much stronger fields, but these can be significantly more expensive. The energy savings may be less than the cost of the magnet.
Also, for a particular application, it may not be possible to get a permanent magnet that is strong enough. In this case the motor must be designed using an electromagnet. The strength of the magnetic field is controlled by the "field current" and the motor speed is controlled by the "armature current".
There is a wealth of information on the 'net regarding DC motors. I've attached a few links for you to follow and read about some of the design constraints and trade-offs for DC motors.
I hope this helps.
Alcoa Technical Center
Try the links in the MadSci Library for more information on Engineering.