Smallparts.com being who they are, I had to order 100 o-rings for my Cub .074 diesel to get one. They are Viton elastopolymer, and resistant to heat and chemical stress.
If anyone wants one, a self-addressed, stamped envelope will net you one (or more, let me know) in return. The 100 only cost $8 with shipping, so this is a freebie!