Almost everything is stored in SIT format or as Toast (= ISO) images.
They are probably compressed with StuffIt 6 or 7.
One option if you have a modern computer is to run Mac OS 8 or 9 within SheepSaver and decompress it there.
Alternatively, if you tell me which sit-file it is, I can download it from Macintosh Garden, decompress it, recompressit with StuffIt 4 and re-upload it.