The Millen motor is fairly old and outdated version of the current motors. The Veloster used by his team was based on the old Hyundai Elantra WRC. Rhys later used a Ford engine from a Swedish engine builder (hint hint). Stephan Verdier had the Hyundai WRC motor in his car which was grossly low in power compared to the new motors yet he managed to do fairly well and it was not easily visible when he was up there with Rhys for a few events.
If Hyundai does a RX program they would run their own motor for sure.