Been too many years since I looked at reel-to-reel recorders/players. but I suggest you study your device a bit more.
In play mode, the speed if the tape past the read head is constant. The movement of the reel motors are such as to keep the slack between the capstan motor and each reel at a constant location. Same in rewind, except the capstan turns really fast.
Reel-to-reel recorders/players with counters count the turns of the capstan/pinch roller, not the turns of the reel motors.
Does your machine already have a counter? if so, you can figure out some way to replace the turns counter with electronic components. If your machine does not have a counter, there may be another model with the counter.
Paul