Engine driven welders are machines that run on gasoline or some other fuel and generate their own power for electric welding. These are often use for field repairs in places where power lines are not available.
In CDDA these can easily be built by installing a small engine with a big alternator, a fuel tank, a battery, controls and of course the vehicle welding rig on a steel frame, you can add a set of caster wheels if you like.
Anyway, I suggest these should spawn as vehicles, similarly to how shopping carts spawn. They could appear in garages, public works buildings, maybe farms and hardware stores.