thats what i thought ...
i have 2 kx500 ..
one in a 93 kx250-frame - great handling..
the other is 88 engine in a 87-frame for classic race.. handles also not bad. the tank-seat-bodywork is smaller than the big tank and seat that came in 88- 2004 (feels little bit like riding a cow, but the best open-class-engine that was ever build
)
a modern frame is the way to go for compeating in the open-class.
but at least it is always the rider that makes a good bike realy fast on track. and it is not important to take an af-frame or put in a ktm-steel-frame.
the point are the better ergonomics , forks and shocks, that will bring any 500 to an higher level of riding...