(defun digits-to-letters ()
  (interactive)

  (goto-char (point-min))

  (save-excursion
    (save-restriction
      (narrow-to-region (search-forward "\\title" nil t)
            (search-forward "\\abstract" nil t))

      (goto-char (point-min))

      (while (search-forward-regexp "\\([0-9]+\\)" nil t)
    (setq digit (match-string 0))

    ;; 1-26 to a-z
    (if (< digit 27)
        (perform-replace digit
                 `((lambda (data count)
                 (string (+ (1- ?a) digit))))
                 t t nil 1 nil (point-min) (point-max))

      ;; 27-52 to aa-az
      (if (and (> digit 26)
           (< digit 53))
          (perform-replace digit
                   `((lambda (data count)
                   (string (+ (1- ?a) (concat "a"
                                  digit)))))
                   t t nil 1 nil (point-min) (point-max))

        ;; 53-104 to ba-bz
        (if (and (> digit 52)
             (< digit 105))
        (perform-replace "\\([0-9]+\\)"
                 `((lambda (data count)
                     (string (+ (1- ?a) (concat "b"
                                digit)))))
                 t t nil 1 nil (point-min) (point-max))
          )))
    ))))

P.S. I have just asked something like this, but for the first 26 digits: How to replace a digit by a letter?.

(defun digits-to-letters ()
  (interactive)

  (goto-char (point-min))

  (save-excursion
    (save-restriction
      (narrow-to-region (search-forward "\\title" nil t)
            (search-forward "\\abstract" nil t))

      (goto-char (point-min))

      (while (search-forward-regexp "\\([0-9]+\\)" nil t)
      (setq digit (match-string 0))

    ;; 1-26 to a-z
    (if (< digit 27)
        (perform-replace digit
                 `((lambda (data count)
                     (string (+ (1- ?a) digit))))
                 t t nil 1 nil (point-min) (point-max))

      ;; 27-52 to aa-az
      (if (and (> digit 26)
           (< digit 53))
           (perform-replace digit
                   `((lambda (data count)
                      (string (+ (1- ?a) (concat "a"
                                                  digit)))))
                   t t nil 1 nil (point-min) (point-max))

        ;; 53-104 to ba-bz
        (if (and (> digit 52)
             (< digit 105))
             (perform-replace "\\([0-9]+\\)"
                 `((lambda (data count)
                     (string (+ (1- ?a) (concat "b"
                                                digit)))))
                 t t nil 1 nil (point-min) (point-max))
          )))
    ))))

P.S. I have just asked something like this, but for the first 26 digits: How to replace a digit by a letter?.

Updates

There is a mistake in the correspodence between digits and letters:

53   -->  ba
...
78   -->  bz
79   -->  ca
...
104  -->  cz 

I have corrected my code as it follows:

(defun digits-to-letters ()
  (interactive)
  
  (goto-char (point-min))
  
  (save-excursion
    (save-restriction
      (narrow-to-region (search-forward "\\title" nil t)
                (search-forward "\\abstract" nil t))
      
      (goto-char (point-min))
      
      (while (search-forward-regexp "\\([0-9]+\\)" nil t)
        (setq digit (string-to-number (match-string 0)))
        (setq sdigit (match-string 0))
    
        ;; 1-26 to a-z
        (if (< digit 27)
        (replace-match (string (+ (1- ?a) digit)))
      
          ;; 27-52 to aa-az
          (if (and (> digit 26)
               (< digit 53))
              (replace-match (concat
                              "a"
                              (string (+ (1- ?a) (- digit 26)))))
        
            ;; 53-78 to ba-bz
            (if (and (> digit 52)
                 (< digit 79))
                 (replace-match (concat
                                 "b"
                                 (string (+ (1- ?a) (- digit 52)))))

              ;; 79-104 to ba-bz
              (if (and (> digit 78)
                   (< digit 105))
                   (replace-match (concat
                                   "c"
                                   (string (+ (1- ?a) (- digit 78)))))

        ))))
    
    ))))

Now it works.