Starter motors require very high currents. Any place in the circuit with resistance a little higher than around 0.001 ohm will cause the motor to not rotate or to turn too slow. Higher resistance points in the main circuit could occur at the battery, the starter B or S terminals, solenoid contacts, brushes, armature, ground from starter case to engine/transmission, and ground from engine/transmission to battery negative. Depends on the design, but in general the most likely spots for high resistance in the S part of the circuit can have is the ignition switch and the clutch/transmission safety switch.
As a practical matter, given the age of your car w/original starter, common sense says to replace it with a new one. You’ll likely get best success by buying an oem version from Nissan. More expensive, but more likely to work the first time out of the box.