The stock mount on my 23 has some type of polymer material, not sure what it is. Probably acetal, HDPE or UHMW. You can usually find small pieces that would be appropriate for a motor mount on ebay. I would suggest UHMW if you have standard woodworking stuff to cut it with. Be careful about chip removal when cutting with a table saw it will bind up the innards in a hurry.
Marine plywood, depending on the type gets that designation as a result of specifications that limit voids in the laminations, surface imperfections, and because it has a waterproof glue used to laminate. That does not make it impervious to water unprotected. If used, it should be sealed with epoxy then coated with the UV resistant coating of your choice.