Simple frequency deviation to voltage converter
One simple method is to add a single-shot / monostable vibrator that is triggered by the oscillator. If the mono time is slightly shorter than the period time of the highest frequency (\$T = \dfrac{1}{60\text{kHz}}\approx 16.7\text{ms}\$), you effectively have a PWM signal that is proportional to the input frequency.

Then all you need is a low pass filter that filters out the high frequency and outputs a DC-voltage, averaged from the PWM.