The smelly exhaust is not typical of Yanmar diesel, once warmed, of course.
My diesels are Shibaura, Mitsubishi and a couple of Yanmar 3TNMs {Polaris Ranger and a Ferris mower}
Bobcat Toolcat 5610G, Deere X744, Cub Cadet IH 982
As far as the Deere x7** tractors are concerned, they did change the Yanmar engine. Pre 2011 engines were 1116cc and 2011 and beyond are 904cc. The bigger engine weighed about 80lbs more too. We have 2 of the older ones in the family (x744 and x749) and they have been very good to us.
The Yanmar gas engine like used in the 322 was a good engine (if not over heated). John deere has used Yanmar engines for a long time in other tractors.
My '91 JD430 has a 3 cylinder Yanmar diesel. Runs great at 1100 hours, and all I've ever done to it is change the glow plugs and the oil and air filters.