Engine development with view to Gas mileage
 

I have a 1980 Volkswagen golf (Rabbit in the USA) with 1588 Gti EG engine. As far as I'm aware, it was the first car in the UK to feature Manual fuel injection. At the time it was faster and more fuel efficient than it's competition. I had one back then and I remember only being able to achieve 38 Miles per UK gallon at the time. (4.546L per gallon). New VW models now feature a 1.4 tsi engine that will average 54.3 MPG in a far heavier car. This engine is more powerful, 124BHP vs 112BHP I know some 35 years has elapsed and engine development has progressed some, but how has it been achieved? The EG engine was noted for it's efficiency at the time and the subsequent 16v heads on following cars could not achieve the same efficiency. (they were not cross-flow) The changes that I can see are that modern car's now have an ECU and the engine has been mapped. How much does this account for? The Tsi is turbocharged, how much does this account for? I want to get the mpg ideally somewhere near today's standards. Can this be achieved. or do I need an engine transplant?

I would appreciate any advice. :)