I am not up to speed on what changes were made when, but there are bassically two types of KX100's, I'll call them old and new. The older ones had on radiator shroud on the right and no shroud on the left, the new ones have shrouds on both sides to look more like a big bike (but still one rad).
The old ones had conventional damper rod forks, the new ones have inverted cartridge forks.
The old ones have steel swingarms, the new ones have aluminum swing arms
The old ones have no powervalve
The new ones have a powervalve (but its rather unefective and prone to failure)
The old ones were bullet proof
The new ones have several different power valve releated problems (still not fixed on the 05's)
The KX100's at my house (I'v ridden both).
- My sister's 1991 KX80 Big Wheel. Mods, RPM 107.2cc motor, ported for low to mid, FMF exhaust.
- My wife's 2003 KX100. Mods bill's exhaust, mabey a 105cc big bore, porting unknown.
Experence
- motor: sister's bike makes more low, mid and top power.
wife's bike is decent, runs very crisp and clean, but isn't quite as strong on the low end as sister's bike
- egronomics:
Sister's bike feels smaller. A bigger rider like myself (5'10" 160lbs) finds the assymetrical tank/shroud set up a little odd.
Wife's Bike feels like a 2/3rds scale KX125. Tank/shrouds are symmetrical which is a plus in my book
- Suspension Forks:
Sisters bike: Forks are damper rod style and do not respond to sharp bumps (like rocks or tree roots) very well, flex is noticable.
Wife's bike: Forks respond to sharp bumps fairly well, flex much less noticeable.
- Suspension Shock: Both bikes feel quite similar, and work reasonably well.
Brakes - both bikes have pretty good brakes, front brake on my wife's bike is a bit stronger than that of my sister's bike.
hope this helps