Going back to the question of UK diesel duty, It's my understanding that the concesion is now garenteed until 2009
As for converting an outboard boat to a sterndrive, I reckon there's a 1001 reasons not to, the whole layout of the structures are possibly different, engine bearers etc, plus the transom could be too thick, for mercruiser at least as they require the transom to be 2.0-2.25 inches thick overall, even an 18 phantom for OCR has a 3 inch transom, so it's probable the Scorpion has a hefty one too! having said all that, I can't imagine having anything but diesel myself as the savings on running costs are massive, more than most petrol heads would ever want to accept or admit to! so I reckon replace with smaller O/B's or sell and get a diesel rig.
Incidentaly, I've run allong side seahound in quite nasty sea's, at speed (60 odd) when it belonged to Chris S, and It's pitch stability is remarkable! a combination of, I think: a 30 footer with loads of warp in the deadrise/ loads of power from very little weight aft, plus the weight of the fwd cabin, quite exellent! Chris tells me that the twin yanmar version was fairly quick but lolopy at lower speeds, not a lot of good to you guys!