I found a good exhaust crush gasket replacement. The vendor is on ebay (mopedfactory). I went by the fact that 1) the listing showed the measurements (38.5mm outer diameter, 5mm thickness), and 2) the description has the text "CF250 172MM" in it:
"Exhaust Gasket Ring for 250cc Water-Cooled CN250 CF250 172MM Helix"
I paid for 2 gaskets, but I got 4 of them. Sorry, can't provide part numbers that yield anything on a google search !
Removing the original gasket was a little difficult, but was able to get it out OK. The new gasket fit perfectly, snug to stay while I installed the header pipe. I installed with the metal side against the port, the black crushable side towards the pipe. It sealed right up.
