77-78 cb750f header is going to fit the best and probably be the cheapest option, then run whatever muffler on it you desire (I'm partial to the supertrapp for sound and appearance).....CONE engineering makes awesome mufflers as well.
another cheap options would be Delkevic. You can buy brand new just the header or their full system.
MotoGPwerks makes a 5-1 yoshimuira replica system, but I've never installed one of those that fit exactly the way it arrived, always needed some bending to fit up.
probably best you can buy would be a Hindle 4-2-1 system from RippleRockRacers, and with current exchange rates (he's in canada) you can get a pretty damn good deal. sound is phenominal, fit is perfect, and it's 321 stainless construction so will stand the test of time.