This recip diesel engine retrofit is not fully optimized in terms of weight. In order to make use of the 120's MGB there is actually a speed increasing gearbox installed between the diesel engine and existing MGB input. The diesel engine output speed is increased and then decreased.
Here is a fairly recent presentation from Airbus Helo on the HCE demonstrator program:
http://www.aerodays2015.com/wp-conte...ierczynski.pdf Interesting technical data on fuel consumption, emissions, and drivetrain dynamics. Also note the relative size of the heat exchanger module on slide 5. Overall a pretty nice job of packaging.