Never had a 550, but you can still get overbore kits on occasion from E-pay.
Cams... have to get a re-grind($300+) possibly use a stock cb650 cam.
Carbs.... unless you go bigger on the piston bore, i'm not too sure how much benefit you can get.
And cb750 carbs are an option i beleive as well.
(cb750 carbs fit on a cb650, so they should on a 550, correct me if i'm wrong)
New suspension would help GREATLY if it still has stock suspension.
I upgraded my rear shocks(ok just new, nothing fancy) DEFINATELY better handling.
Ignition would be a good idea IMO, isn't there a electronic ignition that would work?
Aftermarket coils would be a good upgrade IMO as well.
On my bike when deciding to upgrade/replace parts, I got quotes on all parts i was interested in.
Then based on funds available, bought what I thought was the best at the time for the money I had to spend.
But theres always next winter for more parts...
Oh, wait, this winter!
l8r