Ahh - sorry for the tardy reply, so consider your kart with a driver aboard rolling along at 20mph. The kart and driver weigh - lets say 300lbs. Thats a LOT of kinetic energy to attempt to handle with an 18A motor. Actually thats a really small motor for that application ! (E=1/2(m*v*v)) or approx 5400 KJ or 1.5KW which you have to dump into the battery in the time you need to brake effectively! I would doubt that the motor would have the torque necessary to be an effective brake, but certainly some recharging is possible.
Regenerative braking requires some sophisticated control systems and an appropriate battery system (very high current), although boost caps might be a good alternative to absorb and supply the surge currents. (I have never used them).
The controllers I used for the eTEK motors were from SEVCON and they allowed for regenerative braking at that power level. I'm sure there are other commercial products available now which would be suitable for your kart, but it might be worth looking at the math before spending a lot of ca$h !
I'd love to hear how you get on!