Right, the way you solve this isn't by regulating the cars or their engines themselves (beyond certifying tested performance on synthetic fuels) - instead, you simply regulate the sales of petroleum derived fuel and only allow the sale of synthetic fuels.
In 2035 cars from 2015 will still exist and need to be fueled, let alone cars from 2034. And people driving 20 year old cars can't afford to spend 6x the price for synthetic fuels.
