Answering my own question here - in a word, no. The OEM replacement diaphragm I got from Courtesy fits the Nikki but not the Kyosan because of a design difference in the rod that connects to the pump lever. I also was able to compare the new diaphragm with a 510 Nikki and it looks like the innards could be swapped, as some ebay listings for new 510 pumps suggest. The first photo compares Nikki 510 (L), Nikki replacement, and Kyosan. The difference in rod style between the 2 Nikkis doesn't seem to matter. The second pic shows why the thicker Nikki rod design doesn't fit the Kyosan pump. Reason for all this in the first place is I was hoping to rebuild my non-op Kyosan on the car rather than replacing it. The massive, non-dealer AC compressor bracket blocks getting the pump out without removing all the AC trappings first, including the refrigerant. PITA. Yes, an electric pump is an option being considered.