You might be lucky enough to be able to just replace the brushes, they should be accesable without dissasembling the motor

sometimes the springs that keep the brush in contact with the commutator can get stuck in its little holder.
If one or more of the windings are "burnt out" you will need a replacement motor.
If you do dismantle you can check winding continuity via the commutator (oposite connections to each winding)
A picture or 2 could help identify exactly what motor you need, and if its the same spec as the other logi FF products, I suspect all the logi FF motors pre G25 are very smilar (voltage/current ect)
Ill have a search to see if I can work out exact specs on the motor and where you can get one.
SD.