If you want to keep the B&G Hydra 2000, I think you have two choices:
1.) Drill another hole and install a B&G Paddlewheel sensor. I do not think that you can find a compatible Speed and Depth Sensor that will replace the Depth Sensor and communicate to the B&G Hydra.
2.) Display SOG on the analog B&G cockpit gauges and on the Hydra FFD rather than SOW by installing a NMEA paddlewheel.
The NMEA Paddlewheel device connects to a data port on the GPS, reads the GPS NMEA sentences and finds SOG. It then translates that SOG into B&G language as SOW. If you are a sailing purest, SOW may be important to you if you must know how much current is impacting your SOG, but remember SOG is what really counts and gets you where you are going.
This solution may be the best for you. It is simple to install: 2 wires connect to 12VDC at the Hydra toggle switch, 2 wires connect to NMEA data OUT on your GPS and 2 wires connect to the B&G computer at the same place that sonic speed or paddlewheel connect.
The NMEA Paddlewheel converts NMEA from either a GPS or electromagnetic log to a paddlewheel output suitable for connecting directly to an instrument system, such as a B&G Hydra/Hercules.
Retail price £295.00