swift, 113 bytes

Result

AC

Duration

1172ms

Code [DL]

var x=0
while let l=readLine(){l.map{x=$0>"S" ?0:$0>"J" ?x-49:(x+1)%50;print(x<1 ?"x":$0,terminator:"")}
print()}

stdin

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     K                                            

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stderr

/tmp/code.swift:2:26: warning: result of call to 'map' is unused
while let l=readLine(){l.map{x=$0>"S" ?0:$0>"J" ?x-49:(x+1)%50;print(x<1 ?"x":$0,terminator:"")}
                         ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

strace