David Chivers
As David Davies has tried above I have put Alan Gibson wheels on the existing axles. There is only a matter of a couple of thousands of an inch difference in the diameter but it enough to allow the Gibson wheels to be quite loose.
I hand-knurled the axles along where the wheels will be fitted with a pair of pliers to provide a locking point for the wheels. This was a success.
Secondly they are using a non-standard (short) NEM pocket and as I use Kadee couplings throughout I had to secure the coupling into the pocket with epoxy!