If you can't find that replacement wave washer, let me know, I've got plenty of junk petcocks and they all still have that washer.
A note on the diaphragm. Make sure to go OEM. I've never had good luck with the aftermarket ones, regardless of brand or country of origin. They work at first, but quickly develop a tear allowing fuel to leak.
1990 EX500 - 141,000 miles as of June 30th, 2019
1993 Ducati 900SS - The real canyon carver
2002 EX500 - Undergoing a full restoration