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There are some small 20hp diesel engines out there for small tractors, trouble is they are very heavy, Hatz also makes a number of small diesel engines that would work, or you could just go with a single diesel engine and get great mileage while saving the weight of having a 2nd engine, after all diesels work best for highway cruising.
Yanmar makes some really nice 20hp and larger diesel engines that people have put in cars, but they also cost $5,000 or more for a new one and it looks like $1,500 or more for a used one.
Your math is a bit fuzzy tho, I don't think you will get 120mpg out of a "normal" car at highway speeds no matter what the engine you put in there is, figure your current engine is 15-20% efficient and lets say you are getting 30mpg right now (you don't have a vehicle listed so I don't really know what you are getting) but even at 15% efficient 100% would get you 200mpg and that would be -0- losses, cold exhaust, no radiator and a engine that never gets warm, most efficient diesel engine is a huge one on a freight ship and it tops out at 50%, so even if you could scale it down to fit in your car you would still be getting 100mpg, but that 50% is on a large scale with cylinders that are 3 feet across, so 30% from a well tuned diesel is maybe possible? 60mpg at highway speeds... for that getting an old Diesel VW Rabbit might be your best bet and just keep it simple.
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