American Standard had at least two different flush mechanisms at that time. One was a "tower" that lifted up to flush the bowl, the other tilted. Which style, or a different one, do you have. There is seldom any need to do a "complete" replacement. Just a fill valve and a seal are all that usually have to be done. The "ring seal" at the bottom has nothing to do with leakage into the toilet, but would be a factor if it were leaking onto the floor.