Just one additional word of caution, when unscrewing the dummy plug or actual transducer, be careful that you don't work the thruhull loose. Mine came loose this year and started leaking (not enough to be concerned with), so they have to be rebed that this winter.
I looked at SR's website and all of these parts are still available, except for the head units.
I am going to be replacing my SR mariner knot indicator / log and depth gauge in the spring. The knot indicator works fine and reads accurate (although the log reset button is a little finickey), I just want to replace it with something networkable, but the depth gauge needs serviced (it works, but very unreliable. I think it has moisture in it), and I removed the transducer in favor of the dual frequency sounder attached to my chart plotter. SR Instruments will service the units back to new condition if you send them to them.
Make me an offer if you want them. I can ship them straight to SR to be serviced, they can be sent straight back to you in perfect working condition.