Christy Martin's happy Sydney Sweeney's lacing up her boxing gloves ... saying she's thrilled Syd's playing her in a new biopic, and she wants to make sure her ring moves look authentic.

The boxing legend tells TMZ ... she's excited for Syd's performance, especially because she has a background in MMA and combat sports -- and says the actor's performance can prove dynamite comes in small packages!

Martin maintains she had no say in casting, and while she hasn't spoken to Sweeney yet ... she's anxious to let her know how honored she is she's taking the role.

Christy says she'll definitely be involved with Sydney's boxing training, specifically to help the star with her left hook ... giving her championship-level tools for the flick.

BTW, Christy's unconcerned about people who don't think Sydney's a good actor. In fact, she totally refuted the recent remarks of a certain Hollywood producer, as she told us ... "I think she is young, hot, talented and about to make a movie that in 20+ years, fathers will watch with their daughters to make them aware of domestic violence."

When it comes to her hopes for the script, Christy says it's about more than her boxing journey -- "I want this movie to bring awareness to domestic violence, the challenge of sexuality and overall underdog story. I am a coal miner's daughter from a small town in southern West Virginia that made an impact in a sport that wasn’t taken seriously -- women’s boxing."

In addition to her historic boxing run, Christy survived a murder attempt by her then-husband back in 2010, and later came out as gay and married a woman ... so, there are many, many layers that will be prominent in her story.

As for her on-screen involvement -- Christy thinks a cameo in the flick would be cool, just as a fun nod to her fans.

Like we told you, Sydney's ready for the part ... telling Deadline she's excited to draw on her years of combat training for the new role.