i believe the flange is part of the top tank, soldered in i think. as far as finding cap i'd wait till it comes on ebay or post on wanted for sale page. someone may have a damaged top tank to part out for the flange. good luck.
Is this a J or a U model? they are different.
Part number for the neck is 4F9473. These things are pressed into the top rad tank. Price is about $105. US $. Cheers, Larry.
Sorry Stephen it is a 4U D2. Thank you for the help.👍