12V motor or 24V motor for go-cart?
The voltage of the motor does not determine the torque. A motor manufacture can produce a motor that operates on 12 volts, another one on 24 volts, and the mechanical output can be the same.

But if I were to have the option of selecting between two identical motors, one being 12 volts and the other 24 volts, I would pic the 24 volt motor. For the same power the current will be half. This will allow for thinner wire to be used and control electronics that will not have to switch as high a current. Or the same size of wire can be used and there would be less power loss in the wiring if 24 volts was used.

But another consideration is the available battery. Getting a single 12 volt battery is easy and relatively less expensive. For 24 volt there would be either two 12 volt batteries in series. It adds some complexity but that might be acceptable. Or for example, if a lithium battery pack were available in 24 volts (approximately) with the required capacity, then that might be a good solution.

Basically there are a lot of considerations and tradeoffs that can be made. The whole design needs to be considered. All the pieces. And as I said, the actual motor voltage is not relevant as far as output power delivered. Look at what someone else has done and see if it makes sense, if you can find the equivalent components you need.