A real sport fork sounds good, but it is a lot of work. There are some easier and cheaper ways to improve the front suspension.
        
 
         Better springs:
         
        
https://www.xt660.com/showthread.php?t=19823
         
         
        PLUS
         
         Valves, emulaters or how ever they're called:
         
        
https://www.xt660.com/showthread.php?t=26887
         
        https://www.xt660.com/showthread.php?t=22771
         
         Ciao
        
 Werner