They should put Long John Silver in the original version of Treasure Island in Once Upon A Time. I mean come on, they put not just fairy tales or stories that takes place in medeval times, they also put in Victorian Era classics like Alice in Wonderland, Peter Pan and Frankenstein.
I think plausibly the character from the original novel. Disney had also produced a live adaptation "Treasure Island" in 1950. Also, if Captain Silver and Long John Silver are the same character, he will be probably an equivalent to the live movie and not Treasure Planet.