Wow, now there's a lot of questions. To start, congrats on the new purchase. You are now the owner of the best ride ever made.
There have been very few changes over the last 13 years since your bike was made. Most years, the only change was BNG (bold new graphics). One major change that was a welcome sight came in 2002, when Kawasaki improved the kick start lever. I have broken many of the old levers, but have yet to have a failure on the 2002.
Here is a list of what I beleive is the same from your 1990 to current:
- plastic
- seat
- chain
- sprocket
- swingarm components
- motor
- gas tank
- handle bars
- various other minor items.
- exhaust system
I believe the suspension compenents are not the same, specifically the forks, as I believe the fork diameter has changed, but I am sure someone else can confirm this with specs. If there has been a change since '90 as I believe, this would also lead to a change in tripple clamps.
Essentally, you will find that parts books will list the same parts all the way up to the newest model year. I have seen several 2003 books that didn't even bother to change the year listing, and still lists 2001 as the top, while the parts really fit to 2003 (probably through 200? when you can no longer buy a KX500.
Because of the commonality of parts, I think you will be able to find parts for some time, so you should be able to enjoy your ride for years.
Enjoy...
Rick