Hi, if you have a look on you tube and search Tom sawyers island there are loads of videos giving you a look at the island. You should be able to get a feel for the terrain over there. By the looks of it the raft crossing is accessible but i dont think the island itself is very accessible.
I'm sure one of the cast members working on theisland would be able to let you know which parts you can access, I would have thought parts when you first get off the raft are accessible.