Lots of options for exhaust, as the collector pipe diameter is pretty standard along all the SS bikes and the 851/888 superbikes. Keep in mind, the exhaust can diameter changed in 92 I believe, from 100mm to 110mm. This doesn't really affect anything unless you are piecing a system together from 2 different bikes.
Most of the issues that arise come from the angle of the new cans (i.e. high mounts/mid mounts/low mounts, etc.) and how they are hung from the frame. Also, you're going to want to ask yourself if you want to keep the passenger pegs intact, as high mounts will usually necessitate their removal, although there have been a number of threads on the forum detailing ways to get around this problem.
Lastly, price is going to determine how the exhaust works, sounds, and looks. Prices can range from around $100 ebay chinese cans to ~$800 for Termignoni high mounts that aren't really even made anymore. Coring a set of stock cans is another route, if you're so inclined. It's fun and cheap, but usually a bit of re-jetting will be in order depending on the depth of the core job. This actually goes for any new slip-on that's installed.
My 93 900 came with the same early small diameter A04/B04 mufflers. I have bought another pair off eBay (here in OZ) so they do come up for sale every now and then but the prices now seem a little higher.
The small standard mufflers are a bit louder than the newer ones and I think they fit the bike better - here the later mufflers are marked with a bunch of text, 900/888 and the db spec for the man. I changed back from a pair of louder still polished Duc Performance slip-ons but will be keeping my old pair. I disassembled one of the eBay mufflers and tapped a small dent out of the alloy sleeve. I am happy enough with the rest - that one looked like it had fallen off the bike - the other was fine.
If you are searching eBay you might have to fight off the 851/888 crowd for the early A04/B04 mufflers.
I bought a pair of carbie A14/B14 mufflers because a bit fell out of one of mine (the last baffle piece). With that part out they were far too loud for me. Just be aware that they do fit without any mods but are VERY quiet when compared to the other.