The original version of Sonic #80's cover was Sonic vs. Chaos 0. Instead, this looks to be a redone version of what would've been Knuckles #33. Knuckles, Robotnik, and Big (with Froggy) look as if they have been re-drawn from the original version, while Sonic has been added. Knux, Robo, and Big/Froggy are still in the same areas they were on the original Knuckles #33 cover.
In addition to Rlan's post below, I'd like to add a few things: