I've got a 318, the PS cylinder has a bad leak and a new one is about $250. I would advise against getting a used cylinder and they are not rebuildable. However, I am not sure if the conversion would require a lot of additional parts as I have never paid much attention to my hydraulics.
I would suggest paying a visit to your John Deere dealer and talking to the service department people----they deal with the nitty gritty nuts and bolts for all the models all the time. I'm sure that could save you a lot of trial and error.
