This article is from the ZTerm FAQ, by Les Jones email@example.com with numerous contributions by others.
Yes. The necessary source code for VAX is available via anonymous
FTP from ee.utah.edu in the directory /Comm/Rzsz. You will need
everything but the .tar file.
The source code for UNIX is available via anonymous FTP from
sumex-aim.stanford.edu in the directory info-mac/comm. The four
files are in UNIX shar format. I am told that the code is not ANSI
C. My informant says that UNIX users can use the K & R compiler.