Welcome to the Humminbird Side Imaging Forums JW Simon.
Easiest would be the 1197c SI unit with its external GPS Receiver.
You need to set the NMEA Output menu in the 1197 unit to “On”.
You connect the NMEA output from the GPS “pigtail” wires to whatever adapter cable is needed for the Cobra F55 radio. Depending on what version GPS Receiver you have and what NMEA sentence prefix the Cobra F55 needs will depend on how you connect the two. The NMEA pigtail wire is the second, shorter, set of wires that come out of the connector for the GPS Receiver. The end of the wire is covered with a black plastic cap. Remove the cap and inspect the wires. They will be either Red, White and Black or Green, White and Black. The Black is the NMEA Ground (-) and either the White or Green is the NMEA Output (+) connection you need (try first one than the other).