1981 Lund Royale

Hi everyone. Im new to the site so I thought I would say hello and start a thread about my boat and some questions. I have an 81 Lund Royale. Thats a 18 ft Fiberglass Pleasure boat. It has a 115hp Johnson on it. Does a pretty good job for what I need it. The problem I am having is the floor seems to be rotted out in spots. Also the fiberglass in the rear of the boat is gone due to previous owners letting battery acid get all over. I am thinking the boat actually had a full glass floor. Someone correct me on that if I'm wrong please. I think it was all cut out and a wood floor put in. There are some spots that are very soft and I also think some of the ribbing underneath is bad. How does a guy go about replacing that ribbing, what flooring is the best to put back into it, and what should I do about the fiberglass in the rear that has been eaten away?