Eli-Kart's design has remained largely the same. See how similar they look? However, I also designed and built a large custom brushless motor that could be used to replace the hobby aircraft motor seem in the original design. I used FEA and CAD to help with the design. Here's an exploded rendering of the motor, along with the results of my finite element analysis on the motor using FEMM.
Instructable that details the design of both the go-kart and motor.