The circuit below, when interposed at the red X point on the Figure 2 motor driver circuit, can be used with pendulum lengths of 62.5cm (25") and 2.5m (100"). Select divide by 4 for a 62.5cm pendulum, select divide by 2 for a 2.5m pendulum.[U4 is wired as a "gated pulse generator". It outputs 3 pulses for every one received from the pendulum. U2 divides by 2 or 4 to derive the one-per-second pulses for the clock motor.]
The micropower CMOS ICs add little to the driver's current consumption.
Modify the value of the storage capacitor in Fig 1 per the chart.
Real-world pendulums will likely end up being slightly longer after adjustment.