Andy, that is quite a big boat with a constant deep V hull, but given your location probably very suitable. You might want more gee gees though, 150 hp wouldn't be out of place and something like an Optimax would give excellent fuel figures.
A 1991 big Delta
, wasn't the one that the Royal Marines used to go round UK in by chance?
Don't foget that if it has been rigged for twins then the elephants trunk will be in the middle requiring some modification before a single will work. You might also need an extra long shaft single, rather than cut the transom down.