TIH is a few years away from re powering at the moment. The correct engine size for a Whitby 42, however, is currently being discussed on the Whitby owners group.
This link is to an interesting spreadsheet which is worth downloading if choosing a suitable replacement engine. Please note the posters comments of the veracity, or otherwise, of the spreadsheet and you should check all calculations from another source.
The maximum hull speed for the W42 is 7.67kts. (Hull Speed = 1.34 * (LWL)1/2).
As usual there is no correct answer (but a few incorrect ones) about engine size, 55HP would appear to be on the lower end of the scale with 90HP on the upper end. What is interesting is the jump between 70HP and 90HP only gives you .25kt extra, in heavy seas.
ALL OPINIONS WELCOME!!!