KG is indeed a well-known karting company, this is their website: http://www.kgkarting.it/?lang=en
The 170mm refers to the width and the 5″ or 6″ they are asking for is the diameter, which is the standard diameters for karting wheels. For sprint karting (like you’re doing), 5″ is the regulation size.
For a tire changing device I recommend the KartLift tire changer. Cheap, easy, effective. http://kartlift.com/e-store/index.php?route=product/product&product_id=88