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