If I were you I would go to a Marine store or dealer, buy two of the plastic mounting boards they make "image attached"
Also get a tube of Marine below water grade sealant
cut a piece of that board to size up to cover the big hole, and the small holes
You can use the existing screw holes to fasten the mounting board to the back of your boat, be sure to first apply beads of sealant on the mounting board around the screw holes, and also liberally in a square around the board and zig zag to get a good waterproof seal
On the inside of the boat use RTV blue to seal the nut/bolt/boat surface, no way water is getting in
Later maybe you could add a speedometer or something to that board
Now for the second board, you can repeat the same process on the proper side of your outboard, do not be afraid to drill into your boat, it is not a big deal using bolts and sealant...............the cool thing about the boards is you just have to make holes once, you can swap out transducers, change location up/down left/right if you are not getting the reading you want on your sonar, what sucks about no using a board is you have to make new holes to move it around, the board allows for fine tuning and adding and moving whatever you want
I certainly hope that hole was for something else originally, that would be my guess, it is one of the more ghetto things I have seen on a boat ha ha
Hope that helps, google and youtube about how to get a good seal, buy some good sealant dont be cheap you want it to last, I wish I knew about the boards on my first install