On grid or off grid, you still have to meet the code the AHJ enforces.
If the MG set (electrically driven generator) is connected to the grid, it is a grid tied system anyway, just a separately derived system, similar to going through a transformer. Make sure you follow the SDS rules, mostly grounding issues.
As for the listing, or lack thereof, you may be stuck if you are not willing to have it evaluated. My guess is new equipment might end up being cheaper than that process.
First be sure it was not listed by one of the European NRTLs recognized here (TUV is probably the only one)
The CE mark is pretty meaningless in this discussion. It is a totally different standard.
