If it's useless and access allows, I would break as much of the old transducer away by hand and grind the rest away with a 4.5" angle grinder, using googles and a face mask.
Just be careful to only touch the flowcoat with the wheel.
Anything that will dissolve the epoxy will dissolve your boat.
Another way to tackle Epoxy
Break it down as suggested if it aids access ...then heat an old thin bladed table knife or similar ...(you may be able to bend it if that helps) a gas blow torch is ideal... and it will easily go through the Epoxy
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