Scavenge bits from John's corpse to improve this dismal state of affairs. Be a great friend and make John the Jeep Engine or something.
John's corpse is entirely organic, save for the engine you've already extracted, so scavenging bits of offal from his corpse won't help. Well...it might make a good snack for you, but if you really want to make his current body more fit to house him, you're gonna need to find some tools and some car parts. Which are in rather short supply here.
"Fuck yes, I did it!"
Show off my awesome crab arm. Then also talk to Xan the Engine Spirit.
"Bring us back to the place we came from before this, Xan. I think we might be able to figure out where we are based on that. We had a map, didn't we?"
Also look for the map (just IC).
The engine spirit turns the war beast around and it lumbers back through the forest and into the city outskirts, far beyond the trees and their hidden dangers.
You find the map in the crew quarters, luckily undamaged by the fighting.
Scream at the engine spirit to go ahead and use it's best judgement already. That is, get us to a place where we aren't being attacked by animal people.
Once we're a good distance away, we should put the engine spirit back in the jeep so it can maybe help John.
You disconnect the engine spirit from the controls of the war beast and wrap it around the steering wheel of the jeep.
"I'm not sure what you want me to do here. Your friend is alive and inhabiting the jeep, there's nothing much more I can do to help him, other than speak for him."
Keep assisting in whatever way. Fetching tools? I have a screwdriver taped to one horn.
"This would be easier if I had hands, or if I was bigger! Why'd I have to become such a small beast?"
You watch the going's on, plodding around, ever ready to be useful if you can. For the moment though, things seem quiet, and your help is unneeded, or at least unrequested.