technically, the current Datasmith export in CE 2018.1 is already compatible with UE 4.21. CityEngine 2019.0 (expected in Q2) will have native 4.21 Datasmith export.
The "Model Loader" template project can be used with 4.21 already (via the "switch version" menu item on .uproject file). Unfortunately, there is currently a bug in UE 4.21 which prevents us to port the "CityEngine VR Experience" project to 4.21. We are in contact with Epic and hope to update the templates soon.