Essentially the G series was a tweaked and developed L series engine. The biggest tweak by far was to fit common rail fuel injection to modernise the diesel injection system. Various states of tune were available and just before MG-Rover folded the 16v unit was tested and was destined to produce 160PS. A good writeup about the engine and its development can be found here, as well as good write ups of lots of other rover items!
Allegedly the development engines were sold off rather than crushed, although I’ve not seen any of them crop up in the public domain yet.