The process of replacing the couplers had me scratching my head until I worked out the following:
1. Remove the supplied couplers.
2. Drill a hole of a size suitable for a small coupler mounting screw, hard up against the small rear wall of the coupler box on the van floor.
3. Cut off the small rear wall of the coupler box as per the photo.
4. Carefully cut away the coupler mounting bush from the coupler box (see photo below).
5. Now you need some coupler bushing adaptors from Kadee 20 Series couplers, you are going to use the larger of the small plastic bushes.
6. Assemble the coupler into the box with the bush in the pivot hole of the coupler and screw the lid on.
Kadee 20 Series Adaptor Bushes
Now, the coupler box seems to be a little deep which may help the coupler to droop a little so a 3mm washer needs to be placed on each bogie mounting to raise the van slighty. Adjust the coupler pin with pliers if necessary.
I suspect that the van might be a little low as it now seems to align better to other passenger cars but I have not checked any dimensions.
The Austrains LHO can be similarly modified but it seems to need 1/8" washers as the central post of the bogie mounting is fractionally larger in diameter than the KP.