Hi Rosemary. I went to the Clerk's office for East Feliciana Parish, in CLinton, LA for you today.
There's nothing on their computers (their online records start in the 1960's), so I looked through the Conveyance book and I only ran across one entry for an Isaac Riley (different spelling than the man in your post). But, if he is one in the same as your Issac Reily (I've seen both of those names spelled MANY different ways around here), the one record I found was from Dec. 22, 1870, in Book U, pages 321 - 322, where Cynthia B. Andrews Dunn sold 386 acres of land to Isaac Riley, Isham Riley, and Levi Scott for $2,345.00. This 386 acres was bounded on the North by lands of Mrs. Dave Dunn & Miles Walker; on the East by Mrs. Jane Dunn; on the South by Thomas Andrews; on the West by Mrs. Amanda Woodward. All three men, Isaac, Riley, & Levi signed their "X" mark in the record.
I asked about death certificates older than the ones available online from the State of LA, and was told that they had none stored there unless it was part of a case that came up, or something that was attached to a petition that was filed in court. But I saw nothing under Isaac Riley.
I'm sorry I wasn't able to find more for you, hopefully this tidbit is of some help, if he's your man.
Good luck in your search!!