i'm renovating a 17 ft grp motorboat,?

previous owner has cut away most of cabin to make it look like a speedboat. pl**ker!
i'm not bad working with grp and wood, have all powertools to do a quality job,
what i need to know is best thickness of lloyds marine ply to use to build a new, modern cabin and what percentage of hull depth can i build the cabin? how tall safely in other words. thanks. mark