Actually you would be partially right, I took the YX engine and added parts from the rest of the industry, BBR Taky, Lifan, and YX, also some of my own parts, IE 6 disc clutch etc. I then built two engines and personally took them to the Zongshen factory to develop a new performance engine. The YX engines , the bigger ones were a nightmare and reliability was not one of thhe strong points of those engine. The standard z 155 can first as it was easlier, then the HO engine came about 6 months later. This was early 2008, the general public got the z engine in June 2009 and the HO Jan of 2010.
Zonshen is the second largest manufacturer of small engine like these in China, YX is about 20th in size. Zongshen has a much better manufacturing process. YX has improved quite a bit over the last few years.