The old Atomic Four and it's replacement diesel (from Universal/Westerbeke) will push a C30 along at hull speed. Any more horsepower will just burn up fuel with no real improvement in performance. I'm guessing that the Perkins will have half the fuel economy. The other thing to consider is there is a specification of square feet of airspace per HP needed in the engine compartment for proper cooling and aspiration. Doubling the engine size without increasing the compartment will lead to problems. You could get by with running your blower all the time and putting in an oil cooler, but why? You won't be increasing performance, just making the cabin that much noisier and hotter when running the engine.