So, I'm at a dead end with my ATC 125M. The person I bought it from said it was the original tank, and it looks like it fits, but its actually a metal 200ES tank that was 'clearanced' with a ball-peen on the bottom to get it to sit on the frame (and in the process he screwed up the threads where the fuel petcock goes).
I tried a 200ES replacement plastic tank I have and it wont slide over the frame, the 83-85 ATC 125M tanks are specific, so I'm at my wits end to figure out what to do. They just dont show up on ebay at all. So if someone has one they want to sell, or knows some kind of swap that can happen, I'd be greatful!