I had one of the Kubota assist units on my B8200. It worked fine for a while, but when problems started I put it aside. It had an internal valve and responded according to the pressure caused by the turning of the steering wheel.
I eventually bought the steering valve, cylinder, steering shaft, steering wheel, hoses (the whole unit) from a Toro mower. It took a bit of fore-though and fabrication, but it worked without any problems... and still is.
I had to cut some of the metal with my plasma cutter to set the valve down in, but was only a little bit.
I bought the components from one guy on Ebay for like $225. He parted out mower in Florida.
I'd suggest looking for something similar, maybe one of John Deere's bigger lawn mowers. They have power steering.