Here are a couple of the best diagrams for a resistor cart that I've found.
They are both electrically the same, but each shows different parts of the circuits more clearly.
The wires from the charger receptacle only carry about 20 Amps, so 12Ga (or bigger) is sufficient.
The cable from the solenoid to the wiper arm carries total motor current, so it should be same gauge as rest of high current cables.
EZGO sells replacements, but they are probably 6Ga.
http://www.shopezgo.com/products/Wiper-Contact.html
Give Scottyb a yell.
http://www.cartsunlimited.net/
He will most likely know if anybody has that cable assembly in a heaver gauge or the components needed to build one.