For some long-life vehicles, eg specialist vehicles or buses, replacing the existing engine with one that is of a higher emissions standard (also referred to as “repowering”) may be an option, particularly if the engine needs replacing as part of the maintenance of the vehicle anyway.  This change to your vehicle will need to be certified. Different countries have different approaches to this. In London you need to get a Low Emissions Certificate. In other countries you may be able to have the change made on your vehicle registration papers, or provide other proof for the LEZ authorities.