Note
A simpler, community-maintained, self-contained repository with all steps is now available:
https://github.com/anxkhn/hp1020-driver-mac
It is only ~4 MB compared to ~500+ MB for the Apple driver package used below.
HP never released official drivers for the LaserJet 1020 on modern macOS. This guide patches Apple's HP driver package to bypass the OS version block and uses the compatible LaserJet 1022 PPD.