Not saying this to be snide, but simply take the postwar truck off and replace with the correct prewar version. If you remove the pax car roof you will see how it is held.
They are usually held on top (inside the car) by a little "C" shaped clip, or washer that you bend slightly to remove and then tighten again to re-secure the new truck in place. I would buy an extra (new) washer, or clip, as sometimes they break when performing this task.
Assuming the prewar latch coupler is gone, you will need that part too. It is usually inserted into the slot in the floor and twisted with a needle nose plier to prevent it from falling out.
Ebay is a pretty good parts source. There are always people parting things out. Otherwise Hennings or The Train Tender can probably help you.
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