Actual NJ state statutes
39:3-69 states in part: "...No person shall install or use on the exhaust system of any motor vehicle any device which emits an audible sound unless authorized to do so by the commission. ..."
39:3-70 states: "Every motor vehicle having a combustion motor shall at all times be equipped with a muffler in good working order and in constant operation to prevent excessive or unusual noise and annoying smoke, and no person shall use a muffler cut-out, bypass or similar device upon a motor vehicle on a highway."
39:3-76 states " Every motor vehicle shall be equipped and maintained so that exhaust gases cannot injure any person or animal, and no person shall use any motor vehicle so as to cause or be likely to cause any such injury."
There are also all the environmental statutes that I have not quoted, since some are Federal and all are long.
So you can't have anything like the infamous "whistle tip", cut-out, smoke, or loud mufflers. You can not pipe the exhaust so that it will burn or asphyxiate animals or people as you drive. You also can not change emissions equipment (catalytic converters) unless you follow the federal rules, which prohibit changing any cat in working order for annother, or removing a non-functioning cat without replacing it.
You should be legal if you only changed parts after the final cat, and it is not loud and there is no smoke. There is nothing I could find about having to remain OEM in non-emissions equipment.
Look it all up here:
http://www.njleg.state.nj.us/