Yeah - I agree the (large) Tesla drive unit would tear the little truck apart.
I'd replace the 9 inch motor with an 11 inch, keep the trans, toss a few more batteries in, buy a ready made controller and Bob's your uncle. If you really wanted to get ambitious, you could replace the rear diff, but I wouldn't mess with it.
Use the *small* front drive unit from a Tesla, but now you have a headache for controllers, wiring, deadly voltages, etc. And nobody says the truck can't be front wheel drive...
As an aside,
" The only system that is publicly sorted out for re-use, (Documented) is the Tesla Model S system"
All the CAN messages are documented somewhere?