Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Type
PRIM_ARC_TYPEParameter: Radius
PRIM_ARC_RADIUSParameter: Inner Radius
PRIM_ARC_INNERParameter: Start Angle
PRIM_ARC_STARTParameter: End Angle
PRIM_ARC_END