Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Up Vector
MG_POLY_UPVECTOR
Parameter: Distribution
MG_POLY_MODE_
Parameter: Selection
MG_POLY_SELECTION
Parameter: Enable
MG_POLY_SELECTION_ACTIVE
Parameter: Restrict to Selection
MG_POLY_LIVE_RESTRICT
Parameter: Enable Scaling
MG_POLY_SCALEONPOLY
Parameter: Scale
MG_POLY_SCALEONPOLY_AMOUNT
Parameter: Scale Spline
MG_POLY_SCALEONPOLY_SPLINE