No idea about the in-hull model. I would not just cut away to the second glass layer myself, however. If you insist, replace the foam with good glass.
I like your boat - same as mine 1996, Hull #72. Also, I replaced the wind
speed and direction indicator, then found out afterward that I was experiencing a wire problem (wet connections under mast). You may wish to daylight the wires (eyeball them) before ordering a new transducer. However, you're getting a signal, albeit wrong, and that does suggest your diagnosis is correct.