Is it possible to amend the J&A post award to remove the ceiling or allow for a larger quantity of spares to be ordered? I have looked in FAR 6.303 and it does not mention whether it is possible to amend a J&A.
You are correct— The FAR and DFARS don't address amendments to J&As. However, some agency supplements do. For example, Chapter 4 of the Informational Guidance in the Air Force Supplement to the FAR (see https://acc.dau.mil/CommunityBrowser.aspx?id=483157
) covers procedures for J&A changes. If there is no contravening guidance in the Navy supplement, you should be able to use those procedures.