Welcome Moonie,
Maybe there is another member within a reasonable distance who you could ride with and compare notes.
A very good friend /Riding buddy had a '99 KX5 and I had a CR5 at one time. We swapped bikes every ride.
I got My KX and put it together "stock" and it still scares me

If the bike has never been apart you may consider a leak down & pressure test to give you an indication of the seals. Get yourself a service manual and peruse the site.
Before you know it you'll have the heart of that thing on yer bench getting warmed over and ready for a few more years of service.
Tuck\o/