Hope you get to the bottom of the water issue. I think you will have to have the deck up. Suggest not putting foam back in - not a good idea as a retro fit IMHO. Even factory built twin skin boats can have issues. As for the engine, if it is a 17' Flyer I would suggest a 60 hp. Had one myself before 'Otter' and can honestly say it was a perfect match. Bon chance.