I assume it is a DC motor. You should have DC voltage coming out of the solar cell (For example 10V). You could use a transistor to switch the voltage on and off, then filter the output with an RC filter. You will have to choose the right RC time constant . It will have to be slower than the control pulse frequency, but faster than the desired motor response time.
If you switched the voltage with a 50% duty cycle, then filtered the output, you would get 5V, with a 75% duty cycle, you would get 7.5V, etc. Therefore, you could control the motor by controlling the duty cycle of the switching. A microcontroller like the BASIC Stamp can produce the control pulses.
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