I got a fake copy of Soul Silver without knowing it, and the game crashes when I try to play Voltorb Flip like this question. Is there an alternative way to get coins, or can I never get the prizes?
Not in the Western and Korean Versions.
In the Japanese version, you can trade money for coins. This feature was removed in the Korean and Western versions of the game.
Additionally, the Korean and western releases removed the ability to exchange Pokémon Dollars for Coins.
Via Bulbapedia
If you want those prizes, you’d have to get them through alternative methods. If not found in the game, you can trade Pokémon from other Gen 4 games holding those items onto your game.