Motor control with PWM

I have a Motorized Gimbal that uses a l6205 Chip with 13 rpm gear reduction motor and final drive of 4 to 1. For the venue I am using it in it does not move slow enough. I was thinking of using an arduino board(s) to replace the current board in hopes of better and slower concrole. What boards would you suggest. I have included some of the current board.

Thank you