One way is to replace the potentiometer with a Digital potentiometer. There are many different kinds so start Google and you will find..... If you want a simple to make ( but requires some calibration after it is made) take something from here http://www.maxim-ic.com/DigitalPotentiometers.cfm and replace the old one. The tricky part will be the resolution. If you get 2 or 3 with different values you can connect them in parallell and the combined setting will be used. Bit more trouble but bettet resolution.

If the potentiometer is changing the reference voltage and not the resonance frequency of the reciever you might be able to replace the current potentiometer with an OP amp. Use the pic pwm function to create a DC level, boost it with the OP and off you go. This requires some filters on the PWM output.

There are some potentiometers used in mixers and automation that are a stepengine combined with a pot, these are a bit expencive but a simple solution. Step the engine with the pic and change freq.