I would suggest that you gather evidence of other cars in the series which don't have an OEM chassis or have a modified OEM chassis, then make that public information. This will prove any favouritism.
If none of the cars presently running are modified in that respect then it seems pretty fair tbh.
I'd like to see the car run (just to be clear), but I can see their point if the other competitors don't have the same advantage then it's not a level playing field.