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I don't think it's as complex as a W8 (son owned one ..... neat design.) It appears to be quite similar in spec, aside from size, to the the VW VR6 engine. Most likely (sorry, didn't read all the Horex info) a single cylinder head that covers both banks of 3 cylinders, and a very narrow angle engine .... the VW is/was a 15 degree engine. Are they (VW) still making it .... don't know.
In 1995 VW put their VR6 into the Eurovan, that replaced the 5 cyl. Audi engine in the van. We had one for a number of years. It was the smoothest and most compact engine VW had and made the van what it should have been when they brought it out. Jurgen