I picked up a 300 headpipe off eBay, and then found an early('94?) klx 250 muffler with the removable end cap. It all fit correctly. The p/n stamped on the can is the same as a klx 300.
Working on various sound reducing methods, while retaining the nice low end torque that the "no cap" config. delivers. Will report back in a bit.

 

I didn't know the older 250s had a removable end cap. Thanks for the tip.