So if the 12v is solar power, over what voltage range do you need to operate? It is usual to load the solar cells to the Volatage/Current point at which they are putting out maximum power. Is that something you need, and if so, do you have data from the solar cell suppliers?
If there is no battery, that means that if the motor is not using full power, any excess energy will just be lost. Is that right?
Or do solar cars tend to use maximum motor power nearly all the time?
We might need to know the frequency of the motor PWM, and if regenerative breaking is also going to be an issue.
Richard