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You have to understand that video game development processes changed very rapidly. The Gold/Silver beta is one of a kind. There's no way anything like it exists for gen 3 and on. They wouldn't have full beta sprites and a substantial amount of unfinished pokemon ideas in the actual games. At most you'd be lucky to see one or two scrapped Pokemon. What you're looking for is the original concept art and design documents, not a playable demo.
