No, the plumber got this right. Toilets from around the 90’s are known to be poor flushers. This was when the first wave of water saving toilets came out. They got better over the years to the point where they are now. Older toilets like yours from the pre-water saving era flush great,my toilets' are from the 50's and they flush fine. So I would not trust that kind of plumber telling you its because they are from the 90s. Seeing the work he has done has amounted to shitz A law suit might be in order. I suguest you get plumbing companies that will come out and give a written estimate as to what the prob is and how much to fix. Make sure the quote has parts and labor listed. Good luck!
There are a few variations of the Viper. Might want to post the model number. If you lift the top cover off the tank it should be painted on the top inside wall of the tank.He didn't tell me that and I saw no box or information sheet regarding details! I don't think he knows that!