I have been through this already and I am a 35C owner.
If you have the original masthead unit wtill working after 26 years: congratulations! However, from your second post it sounds as though you may have the more modern masthead unit You will not be able to purchase a new autohelm masthead unit to replace it, although some second hand units may still be found on Ebay.
The Raymarine masthead units for the ST60 onwards are not compatible with the Autohelm display in two important repects. The first is that the signals are slightly different and cause the ST50 display to overstate the true wind by about 20% which in turn upsets the apparent wind setting and any wind display a the chart plotter. Secondly the connector at the masthead is not entirely the same. If you tried to fit a ST60 onwards masthead unit you are likely to bend the connector pins.
Therefore if you wish to change the masthead unit, you will need to replace the cable down the mast and the display. If you change the display, then you will need to find a ST60 display unit which works on the old Seatalk databus (some still appear secondhand on Ebay), not SeatalkNG as this will not work on the Seatalk databus. The ST50 and ST60 can both work together on Seatalk (at least mine do)
The alternative would be to replace with a more modern system working on NMEA2000 or Seatalk NG, but this would be a quite some expense and time.