I've done quite a lot of work on disabled mods for FW aeroplanes and might be able to offer a few pointers.
However, the first thing that needs to be defined is exactly what is the nature of the disability? Which limbs are missing / immobile / limited in their use? What restrictions are there on movement (entry, egress, full movement of limbs, movement of head?)
Next, what does the disabled pilot want to do with the aircraft? Pure private flying?, aerial work?, passenger carriage?
The reasons that these need answering is twofold. Firstly, there ain't no such thing as a standard disability. Secondly, the legal framework is tortuous, and you need to be able to see the best way through it and much of this is relevant to that.
G