Heck, that'll buff right out...
Several have used the Delkevic exhausts on 550 s and they reported pleased results of the install, happy with the sound and quality.
Are the headers solid enough to reuse and could a muffler shop weld up some collectors if you had acquired the parts from Cone Engineering. Then a SuperTrapp or other muffler be fitted?
I do not know if the 900C is a quad muffler setup.
If you cannot talk him into getting rid of the bike, given the general concensus about the 900C being a poor example of a Honda. Now if it were a 900F, I could see to invest in it...
It will be tough to sell the 900C without an exhaust as it is going to not run well until it has some backpressure to even out the exhaust pulses and help with the scavenging.
The Delkevic is as you say, the best option out there, very hard to get an exhaust system without spending $300+ for an aftermarket exhaust. Ripple Rock Racers carries the Hindle, a better system if they make one for the 900C.
Doubtful that they do, would not be a bad idea to ask Kemp if something could be adapted...
David