Most small diesels are either one or two cylinders. A single will provide about 8-10 HP and a twin some where betwen 13 and 18. You want a twin in a 30 footer. With a single you may not be able to get up to hull speed in flat water. If you need to motor into a bit of a sea expect to do about 3 knots.
If you are going to change out the old A4 make it worth while. A twin diesel will give you very good mileage much better that the gas engine.