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Good Afternoon, 
I am new to all of this so please bare with me. I am putting together my materials etc for adding an RP13 based openplotter system in my 1985 Raider 33. The only working electronics on the boat currently are the depth gauge and I will be getting the speed sensor working once I get the barnacles off. I would like to continue to use the old B&G displays but also want them to be input into the RPi so I can set up my handheld HUD and in-cabin screen. So my question is how can I either output data from the RPi to the B&G displays or can I just pigtail the wires from the sensors and directly input them to the displays and the RPi? ANy information would be greatly appreciated. I have a couple of months before she goes back in the water so I have a little time to work all of this out.  Thanks.
It really depends on what protocol your B&G instruments, and you sensors, talk among themselves. Some more recent depth and water speed sensors output standard protocols like NMEA0183 or NMEA2000. Older ones typically are paired with the instrument. For example, my old Raymarine ST60 depth and water speed display needs the sensors plugged directly into the back of the display, and it handles depth and speed detection. I then have a SeaTalk output from the display that needs to be converted to NMEA0183 before it can go into OpenPlotter.

If you are willing to hack up some hardware and software, the speed sensor could probably be connected directly to an Arduino or ESP32 device to detect the pulses from the sensor and convert that to NMEA0183 or even SignalK. I might even have some code around here to do that. Depth is another issue - there's usually some pretty sophisticated software in the instrument that controls the sonar output pings and interprets the echos to calculate depth.

Between the B&G displays, they use "B&G Network Protocol" to communicate between systems. As far as I know, it isn't compatible with anything you can do on OpenPlotter without a translation box.

Perhaps if you can tell us what version of B&G displays you have, we might be able to help find a way to interface to them without breaking the bank.