The Suzuki GS400/425/450/500 was one of the most reliable bikes ever built. I had an '80 GS450L that had 34,000 miles on it when I got it, and had obviously been crashed many times. I put at least another 15,000 miles on it, mostly commuting, but with a couple of 1000 mile trips at freeway speeds thrown in. The ONLY problem I ever had with it was the plastic master cylinder reservoir turned to dust in the AZ sun. I replaced the master cylinder with one from another Japanese bike. It was still running like new when I sold it. All I ever did maintenance wise was change the oil. I never even checked the valves.
The problem with those bikes now, and even back in the mid '90s when I bought that one, which had the stock exhaust, is that there is an almost total lack of parts available for them. I doubt anyone made an aftermarket exhaust for them that long ago. And since they are not considered collector bikes like the SOHC Honda fours, you will find few parts on eBay either.
There may be some degree of parts interchangeability between the different displacement versions of this engine. I would check with the Vintage Japanese Motorcycle Owners Group @ http://www.vjmog.com/
Best of luck with it.