The is one huge issue with that. Moving away and boxing or staying on the outside and winning on points would not be possible. Because the "brawler" who has a fast move forward speed could always just wade right into your chest over and over since your backwards movement is slower.
So it needs to be a static movement speed per fighter. Another words Ali moves fast both forward and backwards same speed. But Hagler moves at a slower speed both forward and backward. This would then give fast opponents speed advantages and force slow power opponents to use the ring to close in on opponents. Which in my eyes is realistic.