Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Type
PRIM_CYCLOID_TYPEParameter: Radius
PRIM_CYCLOID_RADIUSParameter: r
PRIM_CYCLOID_RParameter: a
PRIM_CYCLOID_AParameter: Start Angle
PRIM_CYCLOID_STARTParameter: End Angle
PRIM_CYCLOID_END