Denny - The NAPA part numbers provided earlier by another 'Bolt member are good.
2-4218 is for the pump diaphragm assembly and runs ~$12.00
2-5096 is a rebuild kit containing all gaskets, diaphragm and idle adjustment screw. It runs $19.96.
I don't know what material the diaphragm is made of but I agree with you that Buna-N of the proper thickness should work okay.
I'll be taking my 30+ year old NOS YF apart tomorrow and will let you know what the original diaphragm looks like compared to the new one in the kit.
If you don't have the Carter parts list and service form 5056A let me know and I'll send it to you.
Don
