Can redo a transom from either inside or outside. Don Herman, who does this for a living, prefers to do from outside, leaving about 3" or so of original skin all the way around the sides and bottom so you have original surface to use for fairing the new skin into. (Check Don's site for project pictures at
Hermco Gallery) Probably depends on whether you're just fixing the transom or also want to modify deck, splashwell, etc.