That engine is based off the Suzuki GS and it has different heads. Hyosung has a version of it in its Karion 125RT and it is a DOHC 4-Valve with 14.5hp, I do not believe that these versions at 199cc have a twin cam though only 4-valves. I do not believe that QJ and XY developed that engine in partnership, I would say they likely developed simultaneously.
Since the L Class in the UK has a 125cc and 15 hp limit, XY should have developed the 125cc DOHC IMO. They could have then had a line with that engine for the UK Market, which is a fairly large market.
QJ has both a DOHC 125 and 250 water cooled engines, however the 125cc seems to be limited to KSR? IMO QJ should offer a full line of L bikes with that 125cc.