First thing is to inspect the old screw holes for any water intrusion. If there has been any, drill the holes out to a larger size, large enough that you can insert a q-tip. Mix up a small batch of liquid epoxy and dip the q-tip into it. Push the soaked q-tip into the old screw holes, do this several times over a period of five minutes or until you are satisfied that the epoxy has soaked in and is completly covering the old holes. Put a little of the liquid epoxy on a piece of paper as a test. When the epoxy hardens to sticky consistency, mix up a thick batch of epoxy, that doesn't sag, and fill the holes. You can clean off the epoxy before it sets up with isopropyl After the epoxy hardens (overnight) you can mount your new transducer. If you are using a piece of starboard or wood as a mounting platform, mask off and sand the appropriate area. If you are using wood be sure to seal it on the mounting area and sides (but not on the side that goes to the transom) with liquid epoxy. Mix up another batch of thickened epoxy and apply it to the area that you sanded. You will need to clamp the wood in position, tight enough that it does not move but not so tight that all the epoxy squeezes out. One of those long wood clamps set up to push and braced against a solid object works well. Clean off the excess epoxy and let it harden overnight. I generally try to get the lower edge of the wood along the trailing edge of the hull. You can fair it in after the epoxy hardens. Using the appropriate length fasteners mount your new transducer to the wood/starboard only. You can also paint the wood in a matching hull colour should you wish. I use West System epoxy, but that is a personal choice.