A note of caution - I owned a Ribcraft 6.8m Offshore fitted with a Volvo KAD32 diesel. A nice boat, but definitely stern-heavy because of the weight of the diesel - hard work in a head sea. Waterline length and hull volume is all important when fitting a diesel engine, so small is not necessarily beautiful!
The Mercruiser 1.7L diesel is a popular choice in small RIBs and is certainly lighter than most diesels, but personally I'm not convinced that it's a good engine.
I love my 8.5m Tornado with 250hp Yanmar diesel
, it's a beautifully balanced boat and very economical (5.5 gph @ 32 knots), but if circumstances ever force me to change for a smaller RIB I would go for an outboard installation, to avoid being stern-heavy again.