Thats alot of holes. I would sand down thw entire area to clean glass. Drill out the old holes to clean em uo. Top off the holes with a counter sink bit. Make PB with 1/4" chopped glass and fill holes. If any go thru the hull back off the other side with
some duck tape or let it drip thru, sand, patch, paint/gelcoat.
On the transducer. overdrill size the opening, fill with same pb, drill out to fit transducer, add some glass if you want to, wipe/add some 4200. set trans and set screws with 4200.
If the hull is too thin then add a pc. of wet out ply for height. That will need glassed/PB or 4200 on bottom. Same drill out a described above.
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