They can be, I assume the Alpha and Beta community might be able to help wherever they are on this Forum site for those that post Beta mods might have links that might be still active I guess/hope, otherwise some old modpacks maybe.
The closest to a modpack that has 1.7.3 mods that I know of that might be start is this from this Youtuber I follow:
Site for the above modpack and maybe some that know where to look for mods from that era:
Otherwise there is mods that do Beta like things in modern versions but obviously isn't what your looking for but I thought I'd point it out anyways.
It is tough but it might be possible. I don't know many from the Beta community that use mods but I assume they might know some things if you look around for some and might help you out. Otherwise good luck finding any if you do.