Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Type
PRIM_ARC_TYPE
Parameter: Radius
PRIM_ARC_RADIUS
Parameter: Inner Radius
PRIM_ARC_INNER
Parameter: Start Angle
PRIM_ARC_START
Parameter: End Angle
PRIM_ARC_END