Diesel outboards have of course been discussed on Ribnet before: Big diesel outboard wanted.
This also includes comments from Turbodiesel who I believe was involved in the development/testing of the 'multi fuel' etecs...
To go back to the original question:
Also, in the perfect world of course I would like to do this for no cost, i.e. I find someone who wants to switch from diesel to petrol. Of course I realise this is not necessarily realistic, so what sort of cost implications are likely if we were to switch to say a 50HP 7 year old Diesel engine (I plucked that age/HP out of thin air, the RIB is a HUMBER 5.5m, happy to listen to suggestions)?
as others have said nobody makes a diesel outboard above 35HP (and even those are very rare). Diesel ribs are essentially all inboard engines. Typically the amount of deck space lost to the engine means you will be 6m+ for an inboard diesel (although if you hunt you will find some SR5.4's etc). As a result you will also usually be looking at at least 120HP engines as well.
Of course when the boat is that big the ease of fuelling becomes more important. I'd have thought for a 60HP devising some sort of trolley system for moving fuel tanks would be easier than changing the engine...