(A brief message from the creator) Ok, this is what I can think of which isn't really sci-fi and is a lot more beliveable.
(The episode starts with Mordecai, Rigby and Xeno at the junkyard)
Mordecai: So Xeno, why did you take us here?
Xeno: Meh, you can find lots of cool stuff here, oh look!
(Xeno pulls out a V8 engine from a trash pile)
Xeno: Cool! a V8 that still works! Imagine what could happen if I hook this up to the cart!
Rigby: Hey look! Someone threw away a perfectly good limo!
Xeno: Wait, that isn't just a limo.
(Xeno clears away the junk and it is revealed that the limo is actually part of the remains of Limosaurus without it's head.)
Xeno: I don't believe it!
Mordecai and Rigby: Coooooool!
Xeno: I hope that we can get this running again, it will only take a new head, a couple of spark plugs, a few quarts of oil and a new timing belt by the looks of it!
(A new head, Twenty spark plugs, 200 quarts of oil and a new timing belt later...)
Mordecai: Well this was easy.
Rigby: Xeno, is it ready yet?
Xeno: I don't think so, no matter how many times I rebuild the engine, I keep having leftover parts. Still, I think it would work.
Mordecai: So can we ride it?
Xeno: Look, I don't want you two getting hurt, so I think it's better if I ride it first and see if it is safe.
(Xeno climbs into the drivers seat and starts Limosaurus. Suddenly, Limosaurus starts walking.)
Xeno: Okay, better slam on the brakes before this thing does any REAL damage.
(The brake pedal snaps in half.)
Xeno: Crud.
(Limosaurus walks out of the junkyard and steps on a fast-food joint.)
Mordecai: Aw man, that place made the best chimichangas.
Rigby: At least it was closed.
(Limosaurus walks into the uptown area of the city and soon enough, the cops arrive.)
Cop: Okay whoever you are. stop this instant!
(Xeno looks out of the window.)
Cop: Xeno!? What are you doing there?
Xeno: I accidently started Limosaurus and the brakes broke!
Cop: Well can you steer it away from the donut shop?
Xeno: I'll try!
(Xeno grabs the steering wheel and it snaps in half.)
Xeno: Okay, this is getting ridiculous.
Cop: it's okay! you're heading for condemmation alley!
(Limosaurus demolishes every building that was condemmed, it is heading to the City Hall, a couple of fighter jets arrive.)
Xeno: Those jets are coming to help me right?
Cop: Nope, we had no choice but to call in a aerial strike.
(The 2 jets fire missles at Limosaurus.)
Xeno: Oh man. I hope they could fit Killed by a bunch of missles whilst piloting a giant dinosaur made out of limo's that once almost killed his friend's on my tombstone.
(Suddenly, Mordecai and Rigby are driving the cart towards Limosaurus.)
Mordecai: Hang on Xeno!
(He slams the cart on Limosauruses legs, making it narrowingly avoid the missles and start to walk towards the ocean.)
Cop: Limosaurus is heading away from City Hall, call off the air strike.
(Xeno jumps out of Limosauruses head and lands into the water.)
Xeno: Nice one guys! I owe you one!
Mordecai: You know, Benson's gonna kill us cos the cart's engine is busted.
Xeno: You know, I have a idea.
(A scene is shown that shows Mordecai, Rigby and Xeno riding the cart with the V8 Xeno found earlier replacing the engine.)
Xeno: YEAH! G-FORCE BABY!