Green Power Ranger -- ACCEPTS REUNION INVITE ... I'd Do This For FREE!!!

Green Ranger is COMING BACK -- telling TMZ he's 100% IN for the upcoming "Power Rangers" reunion ... despite the fact some of his co-stars are boycotting.

We spoke to Jason David Frank -- aka Tommy the Green (then White) Ranger -- who tells us he's not only "confirmed" to appear in "Power Rangers: MegaForce" ... he's already on location in New Zealand.

As we previously reported ... Blue, Black and Yellow (#2) have sworn off the reunion, claiming producers were jerks who offered crappy contracts to resurrect their characters.

But Jason -- who's now a pro MMA fighter -- says it shouldn't be about the money, it's all about the fans -- telling us, "I told my fans I'd do [the Power Rangers reunion] for a dollar."

Tommy also drops a MAJOR bombshell about his character -- it's gonna blow your minds!!

BONUS -- WANNA WATCH GREEN DO KARATE IN SOME WEIRD, DARK BACK ALLEY???

Hell yeah you do. Check out the clip ...