Back in the day (1987-1993) I built my own dyno's. Basically I used a Computer power supply for a 5v constant, slave motor, with 3 resistors in the circuit and controlled by 3 switches. Whole thing was about 1" x 3” x 4". Had meter lead receptacles to attach a multi-meter for voltage or amps. I bought the stuff from Radio Shack or another electronics store. Cost was about $15.
RPM was equal to output voltage from the slave motor. Swap the lead and read the amp draw which equaled torque.

I would run it in free mode then flip the switches one at a time and apply the resistive load to see how the motor responded to loads from a start and from a running motor. Worked very well to determine RPM motors and torque motors.
Plus it was great when you could change brushes, springs, timing, etc. Lot more tuning options than with today’s motors.

Thought of building one again, but with so limited tuning options, the G-Force motor analyzer reads timing and RPM plus amps, though does not check loads is sufficient for now.