Exhaust interchange,.,?

Yes other than having to possibly slightly relocate the very rear bolt on position. A quickly cobbled up steel tab on both sides will fix that issue. The bikes are the same other than the back plate where the exhaust bolts up.

Depending on what year C used the engine exhaust stud size may change which will affect collars used up front. Early is 6 mm., later is 8.

Some exhausts bolt onto the oil pan too but you don't have to run that.
I've already told you enough for YOU to make your own decision there. If you are not willing to slightly alter to fit, your mind should be clear what to do, it's not my bike. The biggest issue is to ask the seller if the system has a crossover pipe joining both L&R halves, that indicated the extra oil pan bolts most of the time.

Your biggest risk is the age of it, the systems rusted like you wouldn't believe even with little use, Honda exhausts are well known for it. It can look beautiful and be junk. I myself will not touch used OEM pipes. I put a header on anything I ever build.

That is the 4-2 system used on the 900F. Look just in front of the muffler and the crossover pipe that joins both sides. Right in front of the crossover is a big tab sticking out with the oil pan bolt hole through it on both sides. That hole lines up with a modified oil pan that has bolt holes in it too.

Most of the pipesets do not have that but Honda used it on a few, adding the crossover to later models. Thinking C pipes never did but not 100% on that.

Now that you know what the crossover looks like then you can tell by looking at pics of your C pipes if it has it or the tabs. If not it will be much easier to use C pipes on the K, the only issue is the slightly different holes at back, that's easy to make a bracket for. I can make them up in under 10 minutes.