Here is another place for info. They have plans and parts they sell. And the rails here show how to do 4 wheel independent.
Desert Karts
Alot of mini rails up here use snowmobile engines. You get an easy to ride rail with no shifting. I would suggest a Rotax 670cc if snowmobile route was taken.
I've been thinking about making one myself, for putt putting around cause I got a cb 750 motor sitting in the garage.