My mower has rust in the engine, can it be fixed or must I buy a new one?

My push mower has this shiny copperish colored stuff inside the gas tank. I'm pretty sure it's rust and that it got in the engine. It won't start anymore and I'm thinking it's finally stopped working for good. It's more than 10 years old and my mother won't get a new one.
Murray
3.5 HP Classic
20"
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