555 PWM motor driver controlled by external analog signal
The easiest way to convert a variable voltage to PWM is to feed it and a triangle/sawtooth wave into a voltage comparator.

Obviously, this means it's time to abandon the 555 implementation in favor of a new design altogether. You can get "function generator" chips that will produce the needed triangle wave more easily than you can get anything based on the 555 to do it.

Of course, the modern way to do this is to get a small MCU that has an ADC to read the signal voltage and a timer capable of generating PWM, and program it to do the job.