I would remove the old epoxy. Usually a wood chisel at a shallow angle and one rap with a hammer and it will pop right off in one piece. You might check the face of the puck. I always rough em up with sandpaper (60-100 grit works well) to give the epoxy a chance at bonding to it. I would also rough up the place you are going to glue it down, and then clean with acetone.Be careful about the kind of epoxy you use. You want something that cures hard and slow. Hard so that it will transfer the sound produced by the transducer and slow so that bubbles will migrate out. Really thick epoxy takes extra care to use as if you get any air in it while mixing, those voids will degrade the sonar return data.My approach has been a little complicated, but it has never failed. me. I have always used Tower Hobbies 30 minute epoxy. It takes 30 minutes to set which makes it pretty thin. I usually take a plastic cup that is maybe 1″ larger in diameter than the puck, then cut a “slice” of it off about 1″ thick. Lay it where you want the puck placed, then use some modeling clay to form a bead between it and the hull, all the way around. Mix the epoxy (a significant amount) and pour into the cup slice. Then wet the puck and lower it in to the cup at an angle. Push it to the bottom, slowly move it around to squeeze any bubbles out, then put something on top to weigh it down. Make sure it is still centered in the cup slice and let it be. Next day you can remove the clay and the cup slice if you want. It will be there and stay there, and I always get good sonar returns. Dont forget the weight to hold it down as that will squeeze out almost all of the epoxy except for what is needed to fill any slight surface irregularities…
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