In the midst of a war with a powerful rival, a dying motorcycle gang replaces their recently killed leader with a trusted longtime member who, unknowing to all, has always been, and still is, an undercover informant.

Not really similar, but there's a great early 90's movie called ID. It's about a team of undercover police officers trying to infiltrate british football hooligans during the 80's. The main copper gets in so deep he forgets who he is. Maybe worth a watch.

On the positive side: I like the idea of an undercover informant -- he won't have the same skill set as an undercover cop, plus he can get away with some dirty dealings that a cop would be busted for.
A negative: the "DYING motorcycle gang" makes them sound less threatening to the informant.

This is an interesting situation, Sort of fast and furious but with a motorcycle gang...

However

loglines give us a glimpse of story, not just a situation. For that you need 1) a lead character 2) a goal. 3) Something standing in the way.
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"After being promoted to motorcycle gang leader, an undercover cop is tasked with taking down those he's become closest; without betraying his cover."
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Hope that helped, good luck with this!