Raymarine's currently manufactured Raystar 150 GPS Sensor would be recommended as a replacement for your Raystar 120 GPS Sensor. Please click here to view a FAQ addressing how this sensor may be interfaced to your system.
As the MFD features a NMEA 0183 communications interface, it would certainly be possible to alternatively use a GPS sensor having a 4800 BAUD NMEA 0183 communications interface with the MFD as long at the MFD's NMEA 0183 communications is not currently interfaced to another marine electronics device. Please note that the MFD NMEA 0183 Cable supplied with the C/E-Series Classic MFDs is no longer from Raymarine. The method initially recommended also has the benefit of supplying GPS data to the systems other devices irrespective of the power state of the MFD.
Raymarine products such as its MFDs employ RS-422 for their NMEA 0183 communications. If targeting a NMEA 0183 GPS Sensor, selecting one from a manufacturer specializing in marine should ensure compatibility.