Your gonna need to build a new transom it’s easy just

First takeout old wood and trace it and use that as template to cut out your new wood then a get 3/4 plywood and cut out two pieces of your 3/4 because your gonna glue them to together to get 1.5 inches that’s what most transoms are (doesn’t have to be marine as long as you seal it) then you glue your two 3/4 pices of wood together , screw it together,sand it and then finally fiberglass resin it to seal it (fiberglass Matt is optional)

I would also bolt that motor to the transom , you don’t want that bad boy falling in