All you need is a weak pulse, just basically replacing the energy lost in friction of the pendulum swinging
and if you know the weight and length of string you can calculate the period which wont change no matter how fast its swinging, if you had a stepper motor at the top and gave it a pulse every time its called for it'll work fine
id guess it'd be best to pulse it while its directly below to not notice any jerks or just as it starts to reverse
Not quite true a pendulum's period does depend on its amplitude - most noticeable for larger angles of swing. A pendulum is not a true example of SHM since the restoring force is not proportional to displacement, but to a trigonometric function of displacement.
The formula for the period of small amplitude oscillations depends on the length, the distribution of the mass along the pendulum and the acceleration due to gravity but not the total mass of the pendulum.