Golden Arrow: speed control problem & motor help

I've had a Golden Arrow Frame Buggy for several years now (yes i know, radio shack is cheap crap, but its all i've got). I recently went out and bought a new battery (7.2v 2000mAH Ni-CD) and charger so i could run it again after sitting for several years. charged the battery up and she actually still runs, but there's a slight problem. on the remote next to the speed control lever is a slider which lets me adjust the forward and backward speed at idle (for lack of a better word). with the slider at 0 (we'll say it goes from 10 to -10), the motor is going to full speed now, which is not normal. in order to get the motor to stop going foward, i put the slider all the way towards reverse. that way, i can control the car like i normally would. this still isn't normal, though, and i would appreciate if anyone could point me in the right direction as to how i might fix this problem. i'd like it so that the motor is still when the slider sits at 0 and the speed control, not -10 (full reverse).

i have not rebuilt the motor, but would like to because it is a bit old. i have never rebuilt an electric motor, though, so i need a bit of direction on that as well. i'd consider myself proficient with electronics so i think i can manage. thanks guys