The string you see is a service fingerprint. It contains the responses that were received to the various probes that Nmap sends. If you think there is identifying information in the responses, please submit the fingerprint to the Nmap project to improve detection in the future.
More than likely, what happened is that the service is not sending any useful information. The sample you gave, for instance, does not have a Server:
header that would identify the HTTP server.
To answer the technical problem of how to turn this string:
"HTTP/1\.1\x20503\x20Service\x20Unavailable\r\nContent-Length:\x200\r\n\r\n"
into an unescaped version, you can do this:
>>> print mystring
"HTTP/1\.1\x20503\x20Service\x20Unavailable\r\nContent-Length:\x200\r\n\r\n"
>>> print mystring.decode('string-escape')
"HTTP/1\.1 503 Service Unavailable
Content-Length: 0
"