Is it a 18? I have a 18 and the pitch is about 27.5mm and it is the same as OS FP/LA15.
So, if it's a 18, any mufflers for OS FP/LA15 are compatible with KB18. Brodak stocks it.
http://brodak.com/tongue-muffler-for-fox-35-50th-anniversary.html

Aki

Yes the engine in question is an .18. Not sure about what you mean by "pitch"
My measurement was taken with a pair of Harbor Freight calipers by putting the points of the calipers into the outside edges of the mounting holes.

FWIW To measure center to center distance with calipers and avoid any math  put the  ID measuring ends into one of the holes as if you were going to measure that.
Zero the calipers with it in the hole, then measure the distance from outside of one hole to outside of the other.
The number on the caliper will be the center to center distance.