well for starters kx65 2003 will never sell for 800, to be honest your looking at 500 pushing 600 as kx 85's only go for about 700 - 1000 these days around that age depending on there spec. secondly there is no way that would keep up with a yz 125 as i used to ride a kx65 and they get blown away by kx85's
well nooo...clearly not a fool as i had a kx65 for 4 years and it had a better spec than that. and its not worth 800. tbh pushing 500 tops. and theres no way it would keep up with a yz125 and if it does the yz125 defo needs a new piston