Modern front fender/number plate swap is a good start. some people put the 99-02 rear fender on but it takes a lot of fine tuning/trimming. I personally like the 06 ish 450 front fender but some people run the 2014 one ect, it's personal preference they all fit pretty easily.
If that does not do it for you contact sandblaster and have him build your a KX500 in a aluminum frame