Not one that you'd want to use.
While much of the protocol and networking code can be made common between a client and server, to make a useful general-purpose server requires all sorts of fiddly new code like interacting with OS authentication databases and the like.
A special-purpose SSH server (called Uppity) can now be built from the PuTTY source code, and indeed it is not usable as a general-purpose server; it exists mainly as a test harness.
If someone else wants to use this as a basis for writing a general-purpose SSH server, they'd be perfectly welcome to of course; but we don't have time, and we don't have motivation. The code is available if anyone else wants to try it.