Re: manatee boat
Manatees are really good solid hulls. However, pretty much any fiberglass boat that's been left out exposed to the elements for a long time, as that one clearly has, is going to have rotted stringers and transom. You're looking at a complete rebuild. Do a search on here for stringer replacement. A 5/8" deck if fiberglassed will be plenty strong. 3/4" even more so. You do NOT want the weight of a deck any thicker than that.
As I said, that's a great hull, and if that Johnson has good compression, it could be a great motor as well and good power for that boat. Good luck with your project.