My opinion, the #1 best improvement to this already near perfect machine is the selection of tires and rims that are available now that didn't exist 25 years ago..
Besides a more complex and expensive front end swap as FlyingW has suggest, not that isn't a cool change up, but simpler enhance is a rim/tire change up..particularly on the front.
I have done this with all me ES/SX machines and the change in handling is great.
Here's a pic of my latest..The details for this improvement are available for the asking. No mechanical alterations required..just a bolt on replacement rim/tire and Shazam! A great new look and tremendous handling improvement..

Current toys..
1986 Honda 350X..trail bomb!
1985 Honda 250SX..my main mudder
1985 Honda 250ES..Back in Black Trike
Current non-trike toys:
1990 Honda TRX300FW
1995 Seadoo GTX
1998 Polaris Indy Lite 340(Nearly new looking)
1998 Polaris Touring 500
1998 Club Car (electric)