In the previous reply you said between 36v 1000w vs 48v 1000w, I would go with the 36v as less battery is required. In the second reply, I would go with a 36v brushless motor and over volt to 72v they can handle it, while the brushed you can also over volt but not recommended to go 2x the original specs like the brushless, if its a 48v brushed run it at 72v, either way it seems you will need a 72v controller. Please note this is what I have read, I might have understud wrong. I am also building my first go kart as electric and have been looking for the right electric components like motor, matching controller and pedal throttle I believe is known as pb-6, just like yourself.
I want to work with 24v 750-1000w range motor as it's enought power for my 6-9 yrs old kids, and I picked 24v because I will require less battery and then over volt to 36v, only needing 3 12v battery. Gd luck