You might have some luck.
Most transducers come from Airmar and they are not so different.
For a sounder what matters is the frequency it is made for. Most usual are 200kHz.
From what I can tell Raytheon's ST60 Depth uses the Airmar P19 for depth.
Speed might be even simpler since almost all paddlewheel sensors put out two pulses per rotation and hence are in a band of speed versus pulse frequency that can be adjusted at the display. You have to figure the right wires though. Many speed sensors also have temperature, and they do need supply voltage, so you usually have a common ground, a 12V supply, two independant temperature wires to a PTC and an open collector output for the speed pulses.
I don't know what you plan getting. I know the ST60 pretty well since I fitted a second paddlewheel and some elctronics to average the two before feeding it to the St60 display unit. That was because I was getting over one knot difference depending on which bow I sailed on.
On the ST60 Speed the ground is the shield. Green is the open collector pulse output and red is supply. The temperature sensor is between white and brown.
It is identical to the Airmar ST650 transducer.
Yes I know, boats do have this interesting tendency to love one bow better than the other. But in this case I compared log and GPS SOG in totally slack waters and I did this several times. The only explanation I had was that depending on heel and lift of keel the water speed differs above and below the keel. The paddlewheel is just slightly forward from the forward edge of the keel.
This year I put in the second paddlewheel exactly opposite and I blended the two and now my log seems spot on.