Unless the Readme file by the modeler explicitly says you can take parts,
Just don't do it.
Other sites can be wrong about modelers' rules and modelers' rules can differ from model to model too.
MMD modelers do a lot of hard work making beautiful 3D models that they give away for FREE for people to play with.
The least you-downloader of that model- can do is to read their rules. Reading their rules completely is way less work than the weeks of modeling, texturing, rigging, weight adjustment, physics settings(ugh, physics is a headache and a half) testing and correcting.
Here is the language you need to look out for:
パーツ流用 This refers to trading of parts.
If it's in the section of anything 禁止 that means forbidden.
Now if you see that parts trading phrase under 利用可能事項, then you're possibly allowed. You might have terms like no mixing and matching w/other people's parts and must stay the same character, etc.
If you see something like パーツ利用、他キャラクターへの改造なども許可します then it's definitely allowed.
Hopefully more MMD modelers will include English readmes but if they don't, just enjoy the MMD models for making MMD videos or pictures.
Listening to: Voices in my head