I have a track racing friend who was about 8 years older than me who raced with one and then two knee replacements. At age 70-74, he did a 2:37 2k pursuit and 38.5 500 meters in Manchester to win both events at Masters Track Worlds and BAR. He had great leg speed before and after surgery. And one cannot do a sub 39 500 without a great standing start.
The problem with anecdotes and third hand info is well, it is very unreliable and may not be applicable to your particular situation and may be extreme outliers. If you need the surgery, then you have to go for it and if you are highly motivated to race post surgery, then you will figure out what it takes to achieve that goal.