Hi, I've done similar on my robot, although no kart wheels. I had a collar machined up. Then the sprocket was bored out to fit the collar and welded to it. The collar was then welded to a back plate which was drilled to the wheel stud centres, then using high tensile bolts (not machine screws) I bolted through with spacers to carry the sprocket assembly. If the kart wheels are steel rims not alloy, you could possible just have the collar / sprocket assembly welded directly to the rim. Hope this helps