If you are a Marine, a former Marine or have a very close tie to The Corps using the Eagle, Globe and Anchor device is OK. Other than in these cases I would say Please don't do it.
I sweated, and bleed for that little hunk of metal. It was, and is, a part of the official uniform. It means as much to me as the flag that is always associated with it.
The Leatherneck Tartan is not official in any way, it has never been a part of the uniform. It is perfectly acceptable for anyone to wear. Just be ready for strangers to approach you and ask which unit you served with.