We propose a modified structure of a QDD spectrometer for studying rare andforbidden decay modes of the eta metal. Such a modified structure is based on replacing the quadrupole with a special magnet which provides a quadrupole field along the spectrometer axis and two dipolar field zones extending below and above the beam axis for momentum analyses of decay particles. Particularly, a modified structure of the spectrometer SPES II, that has been in use at the LNS and CERN for more than a decade, is presented. First order and ray-tracing calculations show that the beam optics and momentum resolution of such a modified structure are of a comparable quality to those of the SPES II. In addition, the intrinsic resolution of the special magnet as a momentum analyzer is well below 0.5% and can be realized if the longitudinal extension of the object (reaction vertex) is limited to ±5 mm and its transverse extension (beam spot on target) to ±2 mm.