calc, 92 bytes

Result

AC

Duration

1652ms

Code [DL]

=REPT(LEFT(A1;51);B2/51)&LEFT(A1;C2)&REPT(B3;FIND(A3;A1)-C2),=LEN(A1)-50,=FIND(B3;A1)-B2
T,K

stdin

                                     T            
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
        K                                         

stdout

                                     T            
                                     T            
                                     T            
                                     T            
                                     T            
                                     T            
                                     T            
                                     T            
                                     T            
                                     T            
        KKKKKKKKKKKKKKKKKKKKKKKKKKKKKK

stderr

javaldx: Could not find a Java Runtime Environment!
Warning: failed to read path from javaldx
convert /tmp/code.csv -> /tmp/out/code.csv using filter : Text - txt - csv (StarCalc)

strace