Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Type
PRIM_CYCLOID_TYPE
Parameter: Radius
PRIM_CYCLOID_RADIUS
Parameter: r
PRIM_CYCLOID_R
Parameter: a
PRIM_CYCLOID_A
Parameter: Start Angle
PRIM_CYCLOID_START
Parameter: End Angle
PRIM_CYCLOID_END