Mushy brakes are most times,
1 air in the line. fix- bleed brakes again until no air
2 stock brake line. fix- replace with braided stainless steel line.
If your bent on changing the front master cylinder and plan on using
hand guards go to the Honda master cylinder. The
banjo bolt comes off the back side and it will clear hand guard mounts.
I would start with the cheap way, make sure the brakes are bled properly.
If that does not fix it change to the steel line they don't flex or swell.
Here's how I did mine, it's on a 500 though, toward the end of the thread.
http://www.kxriders.com/forums/index.php/topic,4784.0.html