You've told us nothing about your level of skill or experience — not here and not in your profile — so it's hard to know where to begin.
There are two basic approaches: open-loop and closed-loop.
If your motor's voltage-to-speed characteristic is linear enough over the range of speeds you require, open loop would be the simplest approach. It basically boils down to converting the input frequency to a variable duty cycle digital signal.
There are two simple ways to create a variable duty cycle: you can generate pulses with a constant period and vary the pulse width (called PWM, or Pulse Width Modulation for obvious reasons), or you can hold the pulse width constant and vary the period (called PPM — Pulse Position Modulation). Since you're starting with a variable frequency/period, the latter can be applied directly.
You just need a timer (such as the ubiquitous 555) that is triggered by the rising (or falling) edges of the input signal and has a period slightly shorter than the shortest input period. 87 Hz would imply something on the order of 10 to 11 ms. You calibrate it by varying the pulse width.
On the other hand, if your motor isn't linear enough, you'll have to use closed-loop control, which basically means that you need to measure the actual speed of the motor and adjust its power input to get the exact speed you want for a given input frequency. There are many ways to go about this, but the simplest hardware would be to use a small microcontroller. Let me know if you want me to expand on this.
OK, you basically need a small microcontroller that has three hardware timers. You're going to use one to measure the period of your input signal, one to measure the period of your motor rotation (gotten from something like an opto interrupter), and the third to generate a PWM signal to control the power to the motor.
You measure the input period and apply some math to determine what the corresponding motor period should be. You compare this with the actual measured period of the motor, and use the difference (error) to drive a control algorithm (e.g., pid) that adjusts the power going to the motor.