I've worked in Diesel engine manufacture mainly marine / railway / power generation for 30+ years and it's quite common for engines to manufactured as dual fuel engine running on either diesel or gas. The only design change is to the cylinder head to accomodate a spark plug. As yet I've not seen anything for the newer common rail type configuration, but I suppose it's only a matter of time.I believe a number of Diesel engines have been converted to run on Liquid Natural Gas quite successfully. This was being done by Foden Trucks and Hardstaff Commercials. From comments made by Drivers they are most successful on long Motorway journeys, rather than stop start work.