MAC has some 4-4 systems, dunno if they cover the 650. I do know that the system for a 78 550 is nearly 400 bucks, though
I'd like that link, too. I just bought a set of used pipes that, while not sufferring from as much rot as mine (which are about to go from the inside), are in pretty bad shape on the outside (nicks, dings, and pitting).
Oz Cycle, ATV, Jetski & Salvage (Ottawa, KS) would sell me a used set for $650. I managed to find mine at a "local" shop some 3,000 miles away (in Oregon) for $350. A new set of pipes that was close to the original for $400 would be a steal. Honda wanted $750 for a new set of pipes the last time I looked in 1993 (and when they were still available).
78 550 is precisely my bike. I saw a lot of places selling new MAC exhausts, but only for the 750, as far I could tell (I ithnk that one was a MAC, might've been another manufacturor).