The main problem with a 'motor' is some form of electricity to make it. I suppose if magnets were added and scrap could be refined into copper wires, it would be possible, but logically, it would take ages to make a motor powerful enough to move a raft the theoretical size it is in the game. Even a small one or two square raft wouldn't work. Wood may be light (especially certain types of palm wood), but wood that isn't smooth will create lots of resistance. You'd hardly be able to move at all, let alone fight any strong currents. (I've taken physics classes in high school.) The most logical thought would be sails or the ability to make a single, massive, stationary raft (base) and a few small rafts to take on trips away from the main raft.