It seems that both .sit and .1 files are or were in the past associated as Stuffit compressed archives. As such, your best option to extract/convert any of these would be to use the original software.
Convert StuffIt archive format to Stuffit archive file.
We have found 4 software records in our database eligible for .sit to .1 file format conversion.
Microsoft Windows software - convert sit to 1 on Windows
Popular file archiving utility
Convert from sit file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned sit to 1. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A commercial compression utility
Apple macOS / Mac OS X software - convert sit to 1 on OS X
Mac version of the popular utility
A backup and archive utility