LSPHP 7.4 and 8.0 are available in the Litespeed Repo and you can install them with (eg. Ubuntu) apt-get install lsphp74 lsphp74-common (list the repo contents for all available extensions and just add them in the install command after lsphp-common).
Keep in mind that LSPHP8 has less extensions available compared to LSPHP7.4 at the moment.