Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Stroke
AI_ART_TAG_STROKED
Parameter: Stroke Color
AI_ART_TAG_STROKECOLOR
Parameter: Stroke Width
AI_ART_TAG_STROKEWIDTH
Parameter: Fill
AI_ART_TAG_FILLED
Parameter: Fill Color
AI_ART_TAG_FILLCOLOR