Hi,
The major question here should be, is your rifle pre-ban or post-ban? By "pre" or "post," I mean was your rifle imported into the USA before the 1989 Assault Weapons Ban went into effect. This is the crucial question because if your firearm is "pre-ban," you may add a flash hider without changing anything. However, if your rifle is "post-ban," you may NOT install a flash hider in any circumstance. You may, however install a muzzlebreak, if it is permanently attached, and if your rifle has the correct number of US parts to comply with Section 922r. Your best bet is to talk to a knowledgeable gunsmith, first. Then, if you still do not feel comfortable, contact the ATF. Here's the bottom line, if your rifle is "pre-ban," anything goes. If your rifle is "post-ban," leave it alone until you know the laws inside and out.
Hope this helps,
William
PS By the way, Romanian SAR's are not "pre-ban," they just look that way because they have the correct number of US parts installed. SAR's should NEVER have a bayonet lug, flash hider, or folding stock installed, period.