Motorising a Hornby APT Dummy NDM

My 2021 release Hornby APT will not climb out of my storage yard. It seems like the obvious solution to buy some spare parts, and add a motor to the other NDM. However, it's not possible to order all the parts required, and installation isn't easy.

00:00 -- The problem
01:15 -- Dismantling an NDM
04:46 -- The missing piece you can't order
06:32 -- Making the missing piece
08:12 -- Assembling the newly powered NDM
11:22 -- Testing
15:47 -- Fixing problems
18:33 -- Reassembly fail: body orientation issues
19:43 -- Second test