I have the 3.5 KW unit based on the single
cylinder Kubota, but I can't add much to what Mark said above. It is a compromise for weight and space and it is noisy from the transom exhaust but not inside the boat. The sound shield does a good job.
Looking at the OEM exhaust elbow above I see that the exhaust goes downward from the exhaust port. In some installations, maybe most, this won't allow enough freeboard above the water
line for safe operation without sea water
backing up into the exhaust port.
If I were building a replacement I would ask the fabricator to make a dry riser section that would go upward as high as you can inside the sound shield and maybe higher if you don't use the sound shield. Then two elbows to point downward where the raw water
The newer designed injection elbow on the 3.5KW unit is made this way to avoid these problems that were experienced on the older design which I suspect looks like the picture above.
Take a look at the drawing below for best practices in marine
exhaust system design, courtesy of Tony Athens, Seaboard Marine
and modified to show alternate approaches- flapper, check valve, equalization
line. The two important elements are the 12" spillover height and the spillover point being higher than the muffler
exhaust. Both of these elements make an inherently safe exhaust system.