MSW requires a semi-modern impression. NATO attendees must wear Multicam unless they can muster a squad of 9 or more guys. For example, a group of 9 guys could run a Norwegian, German, UK, etc impression and use the camo patterns in use by those units. I say semi-modern because a group of dudes ran 2010 75th Ranger Regiment loadouts and wore ACU. Obviously, not current modern impression, but close enough.
The vast majority of "milsim" or scenario type events require a green-based camo for one team and a tan-based camo for the other team. Some provide a list of acceptable green or tan based camos that can be used.
You're probably fine wearing USMC 1968 kit since the camo has a green base, but you might get some weird looks from the geardos.