The official proposal is a clarification of the Clean Air Act and “forbids any person to disable, remove, or tamper with emission controls on a certified motor vehicle for purposes of competition.” So it is still possible to turn a standard car into a race car, but it will have to run with emission control systems unaltered.
SEMA officials have already met with EPA leaders and stated that the agency would even make the sale of certain emission-related components for use on converted vehicles illegal. So even if somebody built a race car out of a standard vehicle, it would be illegal for anyone to sell the builder any device that would tamper with emission control devices.
As the history of United States of America has already proven, prohibition only works on paper, and people are going to break the law to do what pleases them. Since the EPA would prohibit the sale of items currently available on the market, the ban would do more harm than good, risking thousands of jobs and upsetting a massive crowd of car enthusiasts.
Currently, the EPA has separate standards for vehicles used exclusively for racing in organized events. However, there are restrictions even for those, as an owner would not be able to drive a race-converted vehicle on the road if it does not meet certain conditions. For example, motorcycles certified by the EPA and built in 2006 and later cannot be used for both recreation and racing purposes after it has been modified and its emissions have increased. The restriction does not exist if the motorcycle has been built in 2005 or earlier.