Perl

echo 'Word: cat' |
perl -lpe '
   1 while s/(?<=Word:)([^a-zA-Z]*)([a-zA-Z]+)/$1 . "*" x length($2)/e;
'

Bash

eval "`echo 'NL=qsq' | tr 'qs' '\047\012'`"; # newline
echo 'Word: cat' |
while IFS= read -r line
do
   var=${line/Word:/Word:"$NL"}
   var1=${var#*"$NL"}     var2=${var%"$NL"*}
   var3=${var1//[a-zA-Z]/*}
   var4=${var2}${var3}
   printf '%s\n' "$line" "$var4"
done