Frustratingly, while motorcycle and HGV/bus engines can usually be modified with retrofits to meet ULEZ/CAZ emissions levels, to modify a car engine to suit is much more difficult as it would require the installation of a diesel particulate filter and associated ECU upgrades. Nothing is impossible, but it's not going to be a simple or economically sensible move. If you have a pre Euro6 car that you want to keep hold of, it's worth writing to the manufacturer for a certificate of conformity to see if the Nitrous Oxide and Particulate Matter emissions fall within the allowed levels to start with - a few drivers have successfully managed to declare their vehicles ULEZ-exempt this way, particularly petrol-powered models.