We are running an older Johnston Sailmaster long prop 6hp on our Cal 28. I feel we are underpowered when coming around into the wind, but it is fine for straight motoring. Hauled us from Half Moon Bay thru the Gate and down to Oyster Point one day in about 8 hours: heavy swells, 12 to 15 foot, close CLOSE intervals... did the job just fine. Only tricky when coming in and out of the slip with a wind off the bow.
The 24 is a lighter vessel, 4000 verses 8000 so I think anything between 6 and 9 HP should be ample.
ain't what ya do, it's the way that ya do it...