If I had to try and map Warpath to 40k, I would probably say that I would likely use the following lists:

Enforcers – Space Marines

Marauders – Orks

GCPS – Imperial Guard

Asterians – Eldar, possibly, sort of

Plague – No direct match

Veermyn – No direct match

Plague – No direct match

Forge Fathers – no direct match

The lists with no direct matches doesn’t mean you can’t use them.  Like Gerry said, there’s scope for just proxying in.  So veer-myn could easily be used for Genestealer Cults, as could plague.  Necrons could use the Asterians list etc.  Forge Fathers could also be used to represent both Imperial Guard and Space Marines having a mixture if heavily power armoured troops and light to medium.  You might have to get creative though.  Also worth noting that firefight isn’t particularly vehicle heavy.  They exist but you wouldn’t expect to see them in the same numbers ot with the same focus as you see in 40k.

I think this is almost a testament to how far Mantic have come, moving out of the shadow of GW and establishing their own world with their own ideas.

It’s a shame people get stuck in the GW rut and don’t seem to want to dip their toe in the pool of other wargames.  Despite still not having rebs (by far the most numerous faction in my collection) I’m sacking off all my Imperial Guard Miniatures (I say “all”, like two squads) and expanding on my GCPS Deadzone stuff for Firefight.  It’s cheap enough that you can do that really, you won’t spend an unreasonable amount of money on Firefight to get to the “standard” size games.  I have the old GCPS Faction booster and starter from the previous edition of Deadzone and a squad from the new version.  I don’t actually need to add a great deal to that, maybe one more squad and a vehicle, to have the equivalent of the Firefight starter box.  I hope you can get your gaming group to maybe give Firefight a try, even using 40k proxies, because while Mantic’s Miniatures haven’t always been the best, their rules are almost always great fun.