Honestly, I have always wanted to see the glorious Jalopy in full softbody glory... I'd make it, but it won't look the best, since HL2 has been optimized further than most games dare. I don't even know if it has an "undercarriage". But I will see what I can do when I'm free.
Undercarriage? What do you mean? Of course it has one, you can see it when you flip the car upside down in Half Life or in Gmod.
I've seen [ I think ] some of those cars and would like to point out that either one or two of them are based off of a trabant 601.
For those who don't know them: Half Life 2 - Scout Car - Build from parts with a eletricity generator engine (still in it's case) Half Life 2 Episode 2 - Muscle Car - A wrecked Dodge Charger, rebuilt with scrap and with a eletricity generator engine too, but this one outside the case Half Life 2 Episode 2 - Jalopy - This was meant to be a rebuilt VW Beetle, but the idea got scraped in favor of the Muscle Car (Thankfully). There isn't a 3D model of this one.
By gas you mean Propane fuelled? The buggy certainly is, since it have the propane cilinder with a tube leading to the engine, but I don't know about the Charger
The charger's engine is the engine. Even when driving it, it shakes and eventually even blows later on
Here is a model of the muscle car, that I found, looks pretty good. https://3dwarehouse.sketchup.com/model.html?id=7849d2aa1334cd1892c505c030dd0b41
From what ive heard, making a mod for BeamNG its much harder than doing it for any other game. So unless you do it yourself, maybe you can convince a modder to take a comission?