I faced the same dilemma a few years ago, and here is the path I took:
The SX120 transducer and the Raymarine transducers operate on different principles--the Raymarine is a three-wire Hall Effect transducer, the Datamarine SX120 is a two-wire small
current device. So you can't really use the existing transducer to drive an ST70 pod, without custom
electronics. And even if you did, you'll be unable to read the result on your S100 Knotlog. However, if you decide to throw out the Datamarine
knotmeter, you will be left with a 4" round hole that will have to be patched or covered up somehow when you install a newer, square instrument.
Assuming you are content with a
SeaTalk 1 network (as opposed to
SeaTalk NG or SeaTalk NT), the following solution will
work. The last Datamarine Corinthian
knotmeter, model CS100, had
NMEA 0183 inputs and outputs, and works well with Raymarine's Seatalk 1 network, provided you use an
NMEA to SeaTalk adapter such as the Raymarine E85001 Interface Box. I replaced my S100 KL with a CS100 on my
boat, and it interfaces very well with my SeaTalk network and my
Garmin chartplotters. I also installed a Datamarine CD400
depth indicator and a Datamarine CW360
wind indicator to add their functions to the network.
Datamarine is essentially out of business, but these NMEA-enabled units appear on eBay from time to time. Also, DMI
Marine, formed by former Datamarine personnel, sells Phoenix knotmeters and
depth indicators which are NMEA-enabled
replacements for the Datamarine Corinthian units.
Hope this helps--at least it's an option that I know works, and doesn't require patching holes in
fiberglass.