Motors are a critical component to this design. The guitar in the YouTube video uses 3 surplus DC gearhead motors (upper left, motor alone, upper right). The motor operating at 12 V and 70 milliamps, under no load, to provide about 120 rpm output after passing through a 64:1 speed reducer. One website stated that these motors provide a stall torque (before the speed reducer) of 80 nMm (815 gm-cm). The terminal resistance is 2.6 ohms.
These motors were purchased from All-Electronics but unfortunately are no longer available. However, applying these specifications to the catalog specifications of motors sold new at Jameco provides a reasonably close match to the Jameco part number 253472 (cross-referenced 38-004) for these specifications: an operating voltage of 12 V and 74 milliamps to provide 120 rpm at with 1100 g- centimeters of torque using a 50:1 gearbox. Currently this motors is about $21 in single piece quantity. it is likely that similar surplus motors can be found. I welcome comments from someone who understands small DC motors.
In the picture, the gearhead shaft has a knurled brass tube epoxied to it because the shaft was a half shaft not well adapted to being a capstan drive. Shrink tubing was applied over this brass tube in the final design.