I've taken this, added autoconf and a portability layer much like (indeed, based heavily on) OpenSSH and produced a portable version. Currently it works on Linux and FreeBSD in addition to OpenBSD. Depending on how well it works out, support for other platforms might be added.
While it's small, it's fully functional for basic sync-to or sync-from requirements, but it doesn't (yet) have the capability to act as a stratum-1 time source. On the other hand, it's an order of magnitude smaller than the reference implementation :-)
$ du -ks /usr/sbin/ntpd /usr/local/sbin/ntpd 328 /usr/sbin/ntpd 36 /usr/local/sbin/ntpd(both compiled normally, dynamically linked and stripped).
Correction: The latter is compiled without optimization, with -O2 the size is 28KB. I also forgot to mention the funny part: the "configure" script is bigger than rest of the source code combined.