The 90 model RM/RMX went to a two piece clutch cover. No biggy in itself, but apparently the cases are also considerably different, so the two-piece clutch cover is a give away to a later motor.
The 91(??) forks are considerably different to the 89, so that's another significant bit of non-compliance.
The 1990 RM125 is one of those "it's OK if you tilt your head and squint" carry over models. I believe the forks are the same as the 89 250's, but there are numerous small differences with the motor - whether they matter, AND anyone can pick them externally is another question.
90-92 KDX200 is a legit carry-over model. 93 and 94 too, if you fit Pre-90 forks.
90+ KX500 is a legit carry-over model if you run kosher Pre-90 swingarm, forks and front brake.