Justin,
I don't know why you want to plow new ground with the Hatz Diesel. I too don't like or trust Kohler but they have two crankshafts on the Lombardini engines taht fit the BCS. One is the straight 1" diameter conventional shaft and the other is a short tapered shaft that bolts right on to the BCS transmission with no adapter flange. The last engine is new at least on the BCS 853 but be warned the 6.5x12 wheels and tires won't fit. Lombardini engines have overall been reliable with some problems with starter, wiring, solenoid and key switch. Service has been iffy as most Kohler dealers have never heard of the diesel.
The other engine in the Yanmar L100 which I love. The added weight of the engine helps with the rotary plow balance, large tillers and definitely with the flail mowers. Yanmar engine have not had any running issues I've encountered and are extremely fuel efficient. Martin Diesel in Ohio is very helpful even early on when the engine first used on the BCS. A fire in shed burned the whole BCS but with new tires it was still being used in Montego Bay being started by rope. Martin Diesel had me all the Yanmar parts within a week.
With two proven engines options for the BCS I'd stick with the ones that are supported. You you really want a Hatz engine look at Subaru engines. They are now buying Hatz diesels and reselling them.