The more modern the better
The 2008 KXF 450 shrouds will bolt on at the tank, but you will need to make a couple of brackets for the two bottom holes. I have a set on my 2005 125, and they look really nice. You can also use 2009 KX 250F fork guards and eliminate the guides that clamp on the forks without having to worry about the guards sticking out. The 2014 fender bolts on with no issues, but you will need to modify the number plate with something to hold it to the triple clamps. The 2005 used posts on the number plate and holes in the clamps. The 2014 is opposite. You could even use a couple of zip ties if you don't want to modify it too much.
The back is another story. You would need to modify a lot of things to get anything newer to bolt on. LundMX has a video on YouTube where he modified an entire later model subframe to fit. It looks trick, but was a lot of work and included welding.