@Phil – Except that the family record, in the Chofetz Chaim’s handwriting, published in one of the early volumes of the Hebrew Meir Einei Yisroel, uses repeatedly the name Poupko. Also a cousin (I believe) a Rav in Flatbush for many years, likewise had that name.
Kagen, on the other hand, is the Russian spelling of Kohen/Cohen (such as Horodna/Gorodna and Horowitz/Gorwitz).