I worked on tons of these bikes and everyone is spot on regarding the power delivery and frame issues. I used to call them "Flexible Flyers". :toothy8: Fun bikes but easy to get into trouble really fast on.
One of the counter guys at the bike shop had a two stroke 400 Yammi twin that I put pipes on, jetted the carbs and installed an aftermarket clutch on. Yellow and black, I can't remember the model for the life of me though. It was some racer replica right from the factory and it was a twitchy as hell wheelie monster. Really light too. Fun bike!