I put 'go kart plans' into Google and came up with a bunch of cool plans, some are $2 and some more.
Here is one:
http://www.mxk.com/1ykplans/ykmenu.htm
Now that you can weld, you could build a kart with a 'live axle' which will keep you from wearing out the single rear wheel drive tire so often.
I built three go-karts as a kid, most of the frames I picked up from advertisements in the paper. I didn't have the welding skill needed to build my own.
Let us know what you decide on.