I have a 1974 30' paceship acadian yawl with a 20 hp Albin AD-2 Diesel. 

Due to a inner seal failure the integrated water pump leaked salt water (Chesapeake Bay) into the motor/crank case where the emulsified water/oil mixture was pumped throughout the motor for 6-7 hours of run time. 

I have pulled the motor to have to professionally diagnosed (what extent is the damage, is it worth fixing?), but am also exploring my options in terms of a repower with a new engine. 

Boat is a full keel, 30' LOA, 21' LWL, 8'6" beam, 7000 lb displacement, 3400 lbs of ballast.

Full specs on boat :  http://www.paceship.org/specs.asp?Model_ID=25

Albin AD2 specs : http://www.albinmotor.com/AlbinMotor/pdf/workshop_manual_ad2.pdf

Would the Beta 20 be a good fit, or would the 16 be equally as effective (I have read here that some of the older engines (my albin is a '74) are more equivalent to smaller modern engines)? 

Is there a big enough cost difference or fuel consumption between the two to warrant making due with a smaller motor vs having the larger motor with some power "in reserve"?

Thanks for any insight! - Ben