I almost fell off my chair laughing about the cat piss stench.
Baking soda and water, as much baking soda as will disolve in hot water...
That will soak up the cat piss stench, only on the keyboard and plastic parts, very lightly, don't put that on the circuitboard or electronic parts.
Usually, cat piss won't get inside, unless its one crafty cat, replace the keyboard if you can and just wipe the thing down a few times with baking soda/water solution.
I got a pc once with this problem, and that fixed it.
You really never know what your getting on ebay I guess, you got a good deal but maybe not after keyboard replaced and so much work to get it unstinky.
Steven