while the R-16 is not as quirky and different than other roadster motors, I'd still stick with knowledgeable vendors vs other engine rebild resources. this will involve some travel inevitably.
There is always Les, and his resources. (can you say "stroker" ?)
there are some beautiful (and recent) examples of rebuilds, and restores, from him.
Troy at Too Intense:
http://www.datsunrestore.com/
I know a few I have seen at Alt Z in HB, but I don't know if Tony takes his motor rebuilds to Troy as well. Tony is a great guy, loves when you bring your own parts, and doesn't do non-essentials unless you ask him. great guy, great work.
http://www.altzinc.com/
(cheesy website)
Dean lists engine resources as well.
http://www.datsunparts.com/1003