Buy the instrument of your choice with the transducer(s) and if the existing hole is a tight fit, use it as is. If the existing hole is too large, then fiberglass up the too big hole and recut another hole to fit the new transducer size. If your existing hole is smaller than the hole needed for the new transducer, then make the hole bigger. To do that, get a hole saw the size of the existing hole and have someone inside the boat place a piece of wood over the hole and hold it in place and then drill out the pilot hole into the wood from outside the boat. A long piece of wood works best because your inside person can place each of his/her two feet at the ends of the wood with no worry that it will move because pressure will be applied to hold it in place, and not worry about drilling a hole in a foot. With the wood inside still in place and secured with the pressure of the feet of your inside person, you can use that pilot hole to drill out the larger hole saw size. Obviously the boat has to be out of the water and also you need to use the appropriate caulk to bed the new transducer(s).