Hi,
What was there before? An A4? If so, it has a prop that was designed for that engine (30 hp, IIRC). Any diesel will have a different transmission ratio and require a re-prop, too. Some A4 skippers installed a better prop, but astill won't work with a diesel.
Fresh or salt water? Freshwater cooled or raw water cooled?
Do you have an engine in mind, or two?
Our 34 footer has a 21 hp, 3 cylinder Universal (Kubota block), freshwater cooled (i.e., heat exchanger). Later models has 23 then 30 hp engines, 3 cylinder, then four.
Where do you intend to sail? Lakes would get away with two, but anywhere else you'd need at least three.
The Cal 34 is a heavy boat.
We need more info to help you out.
Good luck, get back to us.