Re: How much deck over the cabin do I remove?
I think I would start thinking look for seams. You might take off a much bigger piece than necessary but if there is a natural seam, perhaps along the seam of the non-skid or something like that, so you better hide the seam. Keep in mind as you do your work the value of the boat. Don't go overboard as far as cosmetics goes as you will see little increase in the value of the boat. Take lots of pictures too so you can show prospective buyers that you did it right. If you can't find a natural seam, perhaps you can make one, covering it with something like Kiwi Grip instead as it is thick and textured and will hide a multitude of sins.
If it is plywood, you may have to do the whole deck, as water tends to weep along the layers. Search some of the threads here as well I believe tommays had some good photos of his work, and his boat looks really good afterwards so he has some good advice.