You have to re-prop or you'll do in your new engine because it's running at half the rpm and won't be able to get up to nearly full load with the A-4 prop which went at twice the rpm. Diesels need to be able to run at power or they have problems. You A-4 prop was probably the stock 12" dia. by 6" pitch.
Since the Universal diesel's a common replacement ask other Cal 29 owners and see what they're using and how it works. If you're looking at a fixed prop go to the Michigan Wheel website and send in a request for a prop calculation.
Michigan Wheel asks for rated horse power (not shaft horsepower which is about 90-95% rated BHP). So I think that their calcs will be on the conservative side. If you're reeeely into it you can do the calcs using Dave Spurr's Propeller Handbook as a check on Michigan Wheel
I'm repowering my Cal 29 with a Beta 16 the data I used for prop calcs:
Displacement 9500# (1500" greater that the base boat to account for gear)
Speed at power 6 kts
Shaft Horse Power: 15
Beam at the waterline: 8'
Prop depth 2'
Max diameter 13”
I've asked around and other Cal 29s have put in 13" props successfully. The issue is that the slower moving shaft wants more blade area (bigger diameter)to keep from cavitating at power.
The decision you'll have to make is whether you want two blades or three.
Michigan Wheel came up with a 13R9 for a 2 blade and a 13R8 for a 3 blade using thier Sailer model props
My check with Spurr's methods lean towards the 3 blade at a 9 in pitch and that the 2 blade would be close to cavitation. But I don't know how it will work yet as I'm still putting in the engine. Once you get the prop in if it's off you can take it to a prop shop and that can correct the pitch to be right on.
All this applies to a fixed prop. For feathering props check with the manufacturer. You can adjust the pitch on these props yourself.