I don't think any aftermarket companies currently make a stock looking set for the 75. Most companies haven't made them exhausts for any XS650 in 25 years or more.
There are companies that make mufflers of various styles that can be put on a 75.
On the earlier bikes the head pipe has an inner and outer pipe that was sealed together. The muffler just slipped on and sealed with a nut and gasket.
easy to remove, just unhook from rear foot peg mount and wiggle a bit and they slip off.
Now you can slip on most any muffler. The outer head pipe is about 1.5 inches in diameter, so get a muffler of this inlet size or the adapters to clamp a 1.75 muffler on.
I also recommend replacing the crappy clamps that come with the new mufflers with Harley 1.75 clamps. A very strong clamp for about $10 each.
Mike's as well as other places have a Dunstall replica muffler that looks, sounds and runs well for a not to expensive cost.
I built my own headers and run some shorty megaphones, runs well look ok and weigh tons less than stock.
Leo