Yes, there are a lot of DIY kits, but junkyard takeoffs are a fraction of the price and easy to replace
I would stay away from US made EPAS systems (GM stuff), because unlike JDM units, when their controller fails the electric assist is disabled, and on JDM units (Toyota for example) if the controller fails the electric assists stays at full
As for potentiometer and JDM units, I have found the following:
1. they take VSS, and adjust effort according to the controller logic, usually the effort increases up to 80km/h, and then stays the same. You have to be careful when selecting the OEM takeoff because all new ones use CAN to transfer data, so figuring that out is going to be a bitch. Old Yaris, Corolla and some other pre MY07 cars use regular ones that will work fine
2. Controller looks at the difference between the torque sensor and steering column rotation sensor, and applies assist accordingly. This one is tricky to modify
this article talks about this mod but its not exactly what I was after
Ideally I want to map out a signal from my ECU to have the assist I require at a given speed, but I don't see how to do this with the JDM takeoffs yet....