USD forks arrived in 1993.
Plastics are the same colour all the way from 89-94, only the colour of the seat cover (darker blue vs light blue) and the graphics changed.
Haven't heard about the muffler thing.
The four other changes I know about:
1. The 89 models have a captive nut on the top shock mount. 90+ are a loose nut.
2. The 89 models have a slightly narrower suspension linkage (geometry is the same).
3. The 89 models have 8mm bolts in the lower & front engine mounts, later models use 10mm bolts.
4. The 90+ models have a voltage regulator on the top of the airbox to avoid the blown headlight globes of the 89 models.
Maybe a bow tie wearing pedant may argue that this is different enough to stop the 90-92 models being carry-over models, but #1, #3 and #4 are easy mods. #2 would be damn difficult to prove unless the scrutineer had both bikes on hand to measure...
I've also read that #2 & #3 were running changes that came in mid-89, but haven't been able to confirm it either way.
