You only need the cats for a "street legal" vehicle. I know people who don't have cats, and here in California. We have the option of registering our vehicles as "non operational". That means we do not drive it on any public street. Then there is no need for a smog check. What does that really mean? That means the vehicle is only for "off road" use. Some people drive on private properties like ranches, dirt tracks, resort parks, et cetera. For years, my family has kept an old Toyota Pre-Runner and a Jeep Wrangler where we hunt, and both of those are not "street legal".