import re
text = 'local Mission = {\n start_done = false\n game_over = false\n\n} --End Mission'
rx = r'Mission = {.*?} --End Mission'
print(re.sub(rx, lambda x: x.group().replace('false', 'false, \n'), text, flags=re.S))
aW1wb3J0IHJlCnRleHQgPSAnbG9jYWwgTWlzc2lvbiA9IHtcbiAgICBzdGFydF9kb25lID0gZmFsc2VcbiAgICBnYW1lX292ZXIgPSBmYWxzZVxuXG59IC0tRW5kIE1pc3Npb24nCnJ4ID0gcidNaXNzaW9uID0gey4qP30gLS1FbmQgTWlzc2lvbicKcHJpbnQocmUuc3ViKHJ4LCBsYW1iZGEgeDogeC5ncm91cCgpLnJlcGxhY2UoJ2ZhbHNlJywgJ2ZhbHNlLCBcbicpLCB0ZXh0LCBmbGFncz1yZS5TKSk=