It could be a team effort. Amy with her determination, Tails with his intelligence, and Knuckles with his strength could all come together. They might search different places for the cure ingredients. Amy could search in the forest for a rare flower, Tails could look in his lab for a chemical compound, and Knuckles could dig for a special mineral. All these combined might save Sonic.
There's also the possibility that an unexpected character saves Sonic. Maybe a new character shows up who has a natural immunity to the poison. This character could share their immunity or knowledge with Sonic's friends. However, it's also likely that Sonic himself, with his strong will, manages to fight off the poison to some extent while his friends are searching for a proper cure. His speed could also play a role in slowing down the poison's effects, buying his friends more time to find the antidote.
Well, Sonic might start to lose his speed gradually as the poison takes effect. His friends would surely rally around him, with Tails using his intelligence to try and find an antidote. Maybe they'd have to race against time to find a special ingredient in a far - off land that could save Sonic.
His friends like Tails and Knuckles are likely to try and save Sonic. Tails is smart and could come up with a clever plan, while Knuckles has the strength to fight off Scourge if needed.
It could be Dr. Eggman. He's always been after Sonic and trying to defeat him. Experimenting on Sonic could be his new way to find Sonic's weaknesses or to gain his powers for himself.
Obviously, Sonic and Metal Sonic are the main characters. Sonic is the one being framed, and Metal Sonic is the one doing the framing.
Well, Sonic Tails could save Sonic fanfiction by being a central figure that unifies different storylines. He has a strong connection with Sonic and other characters. Fanfiction writers can build around his relationships, his loyalty to Sonic, and his own growth as a character. His character traits can be explored in various ways, like his courage in the face of challenges. By being such a rich character to write about, he gives new life to Sonic fanfiction, preventing it from fading away.
In this fanfiction, the Chaotix detective agency could also play a role in taking care of Sonic. Vector might not be the best at medical stuff, but he could use his detective skills to find out the source of Sonic's illness. Espio could use his stealth to keep an eye on Sonic's surroundings, and Charmy could keep Sonic's spirits up with his energetic personality.
It could be one of Sonic's old enemies who found a way to transform into him. Maybe Dr. Eggman created a sophisticated robot that looks and acts like Sonic.
I'm not sure specifically as there could be various interpretations in such a fanfiction. Maybe Metal Sonic sets Sonic up in a way that makes Sonic look bad to the other characters, perhaps by faking some evidence or causing chaos and making it seem like Sonic's fault.
Sakura would be a likely candidate. As a skilled medical ninja, she has the knowledge and ability to diagnose the poison and attempt to create an antidote.
You might be able to save Sonic by finding a powerful item or ally within the comic's story.