Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Start Radius
PRIM_HELIX_RADIUS1
Parameter: Start Angle
PRIM_HELIX_START
Parameter: End Radius
PRIM_HELIX_RADIUS2
Parameter: End Angle
PRIM_HELIX_END
Parameter: Radial Bias
PRIM_HELIX_RADIALBIAS
Parameter: Height
PRIM_HELIX_HEIGHT
Parameter: Height Bias
PRIM_HELIX_HEIGHTBIAS
Parameter: Subdivision
PRIM_HELIX_SUB