Harry, I don't know about the 91, but as far as chain slack goes, I don't think it would be any different than my 2002 KDX200.
1. Put bike on it's kick stand. ( just as if you were parking it outside the pub).
2. Measure from top of swingarm to bottom of chain. This is done midway on the chain. On my bike midway is centered between the two bolts on the kickstand mounting bracket.
3. Chain slack should be 55-70mm. I would go halfway at 62mm unless you are in muddy conditions. If muddy, then go closer to 70mm. If your chain is stretching more lately, then I would go closer to 55mm if you are going on a long ride. If it is stretching more than usual lately then you probably need new chain and sprockets soon
Enjoy the bike! :thumb:
I took a long hiatus from trail bikes too, until last year. First bike in 17 yrs. I don't know why I waited so long to get back into trail riding!