Re Differential (provide Link) OK but then the trike frame has to be built to use it, & that fabrication at US overhead rates wont come Cheap.
I dont perceive the OP is ponying up a couple Large for a Custom made Machine, Perhaps they are ,
as they have a son who will have decades ahead of them to Use this machine.
Perhaps Clifford Knows Fabrication shops specializing in building equipment for special Needs Clients and can share that resource ?