welcome to the forum!
there are instructions on how to build a kart from scratch 100 times over in this forum. just takes a little searching and some reading. youtube is also a great place to search for "how to build a go-kart", and other popular search phrases.
the basics for attaching the engine to the wheel is: wheels must be connected solid to the axle (1" solid axle with 1/4" keyway running the length of the axle, as well as threaded ends for locking nuts) via 1/4" key and locking nut (to keep the wheels from falling off), as well as a large sprocket (60+ tooth) on the axle for the chain. axle will be mounted via bearings and bearing hangers to the go-kart frame. engine will be mounted to the frame, with a clutch of some sort on it (TAV2 is a popular clutch for 8 hp and under. for a small kid kart you can use a centrifugal clutch, but a torque converter clutch like the TAV2 is always better). chain runs from the clutch to the sprocket on the axle. chain turns sprocket, sprocket turns axle, axle turns wheel. yee-haw!
use the speed calculator to determine what your top speed is going to be, here:
http://www.diygokarts.com/speed-calculator.html
more speed = less torque. a good balance for the 212cc motors is 18" tires with a 72 tooth axle sprocket and 12 tooth clutch sprocket, in my opinion. use 41 chain and 41 sprockets, not 40 or 35.