Gammera, the Invincible (1966)
Edited US version of the original Japanese monster classic "GAMERA", featuring new footage of American actors, including Dick O'Neill (THE JERK). MST3K riffed the original version, and honestly I'm not really on board for re-riffing movies featured on that show, but I do like Mike, Kevin, and Bill very much. I would love to see their take on this alternate version at the least. I also think this version may be in the Public Domain, so perhaps a VOD.
Toyland Chairman commented
It's been riffed. Only the second of my many requests to be riffed, after ATTACK FROM SPACE (1965).
Robert Christopher Fagnan commented
Also features inserted footage of a guy who looks like Lewis Black arguing with another guy on tv over the existence of Gamera and flying saucers which devolves rather quickly; must be seen to be appreciated.
This one is ranking WAY too low. Let me add to what the original poster had to say. Gammera the Invincible offers a lot. There's the nostalgia for the MST3K version, that certainly helped with the recent Manos and Santa Claus releases/live shows. BUT- this is a completely new edit/dub/script from what MST riffed. New material + nostalgia. That sound you hear is hipsters drooling at that sentence. The film itself adds scenes with American actors in the same way the US version of Godzilla did. But in place of a stoic Raymond Burr you have an obviously drunk Brian Donlevy. Donlevy's slurring is reason enough to riff this one. Much like the original Gamera, it feels like there's room to riff, The new scenes add a lot, but not all of it good. Well, good for your purposes. And as stated above, I also believe this is in the public domain, but you have people to research that. I think it's perfect Rifftrax material. It's on Youtube, give it a spin and see if we're right.