First, you don't have to limit the current; the motor draws only as much current it needs. So you want two separate circuit functions, one to regulate the voltage to no more than 12 V; and one to monitor the voltage to see if it dips below 12 V.
When the monitor circuit disconnects the pump from the PV (photo-voltaic, or solar panel), the panel voltage will rise above 12 V. If the monitor then reconnects the pump, the voltage will drop below 12 V again and the system will cycle like this continuously until there is enough light to run the pump correctly. This oscillation is bad for the pump. Think about a minimum off time to reduce this oscillation on cloudy days. One minute, ten minutes, whatever; since we don't know what the pump is doing, we can't recommend anything.
What is the maximum output voltage of the PV on a sunny day with nothing connected to it?
Where are you located?
What is your skill set for building a small circuit with a heatsink?
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