node, 119 bytes

Result

AC

Duration

915ms

Code [DL]

a=require("fs").readFileSync(0);for(i=0;i+51<a.length;)a[i+51]|=a[i++];for(;a[++i]!=116;)a[i]|=a[i-1];console.log(""+a)

stdin

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   K                                              

stdout

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   kkkkkkkkkkkkkkt                                

stderr

strace