You should have little trouble replacing the ST50 with a newer
Raymarine ST60. The Seatalk bus is the same, but I believe the plug is
a little different. You can splice a Seatalk cable with the new
connector to your old cabling, and it should work fine. The biggest
problem may be aesthetic, what with the different colored case...

Aaron Tinling
s/v Sweet Destiny
www.navagear.com