Recently I found that changing the partType attribute in Overload can lead to some interesting things.
How to get old props
For example, by grabbing a prop and looking at its part type, it will say Engine-Prop-5 if you grab a T1000. We know there is only three props currently available, so what happens if we change it to Engine-Prop-1, or Engine-Prop-2? You will get an old engine. It should say at the top of screen, partType has been changed, all connections are destroyed. That means it worked. Congrats, you have an old engine. Then save it as a sub.
How to get passengers
Change the partType of anything to ‘Passenger-1’.
I hope this was useful. Have fun.