Adventure Island isn't just a blatant ripoff, it's literally the exact same game but with a different main character. But it wasn't an illegal ripoff a la Great Giana Sisters; Hudson acquired the rights to the game.
As for the game, it's way too basic and repetitive compared to other platformers even of its day. I'd give it something more like a 4/10 if I had to score it.
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The Temple of Kraden, a forum for fans of Golden Sun:
http://z9.invisionfree.com/The_Temple_of_Kraden/index.php?act=site