A 240D mercedes engine is about the size of an A engine, 60 or so hp, nice heavy iron ---but I think a 190D 2.2 would give the best fuel mileage,in the 190 car with 5 speed they got up to 50mpg , a 2.5 turbo would be a good fit, an 80s Peugot diesel wagon had a closed driveshaft.
The WWII diesel conversions were not real diesels, they heated the fuel very hot, and the carb was heated to vaporize the heavy fuel, actually they were dual fuel, started on gasoline ---I have even seen a Packard that was converted, there was a small 1 gal tank attached to the radiator rods, long shafts for the valves to the dash--2 headgaskets for lower compression-the owner was a boat captain, gas was rationed, fuel for diesel boat was no problem---this conversion tended to be hard on the engine, i later heard that the valve seats were all cracked on the packard( I was 32$ low on my sealed bid, it still was a good deal at 2255$)
i saw a picture of a conversion done to an A, the fuel pipe was wrapped several turns around the ex manifold, there was a box around the carb for heat---I guess a tillotson wasn't very good for this conversion
I have 3 older S class benzes, 2 are diesel, 1 gas, gas 14mpg, diesel 25mpg ---on the road --gas 18 mpg, diesel 32 mpg----an A with a 2.5 turbo would probably get 35mpg at 60-70