If you plan on racing 4 cycle engines.. Buy a kart designed for 4 cycle engines. Don't buy a kart that was built for 2 cycle and then worry about how to make it adapt to your 4 cycle platform. Determin what type of seat you want to use. Layback/oval style or the situp 2 cycle style. Some chassis, it is a struggle to get a layback seat in them some chassis you cannot put the situp style seat in easily. Many are running 4 cycle karts with situp type seats with their Animal, LO-206 or Clone motors on them.
Shop for a purpose built 4 cycle chassis, Margay, Coyote, MGM, Bandit, Razor to name a few, they all build 4 cycle chassis. Get the model and serial number off the kart and contact the manufacture or an authorized dealer to find out about the kart. I am sure where ever you live you can find one in your area and price range.