Good luck getting in touch with Fatar. Might try calling Sweetwater instead.
I don't believe the Fatar TP-10 is available anymore. The original had lead in the hammers, so they became "illegal". Doesn't make a ton of sense to me since lead is still manufactured and sold for other purposes, but what can ya do. The lead was inside the hammers under the keys -- inaccessible during normal use, FWIW.
The K2500X/S has this same action. You really have to search for used or new-old stock to find replacements.
Kind of a rabbit trail, but...
I believe the K2600X/S uses the TP-10MDF action. There are some differences, one being the underside of the key. The "finger" arrangement (which pivots the hammer) is different. Also, I believe the lead was replaced with steel. The K2600X/S action should work, however, Fatar no longer makes this model action either, and the likes of Sweetwater and Kurzweil can't even get a replacement key for any older PC or K series board. Same case for older SP series pianos. Basically, unless you have a current model Kurzweil, you can't get action parts for it. I found a parted out K2600X and hoarded a set of keys, but lack hammers. No problems yet... We'll see. The PC88 and K2500 hammers were very weak, but my K2600 seems fine after about 12 years. Anyway, I'm rambling and somewhat off the subject.
To address your actual question, I don't think the TP-40 action would work. Mechanically, it would probably fit, but I have heard an account of someone dropping a TP-40 in to their K2600X, which as stated above, is designed for the TP-10MDF. The result was mismatched velocity readings between the black and white keys. The processor just wasn't designed to handle the differing signals the keyswitches were sending. I don't think there was a fix for this. Maybe you could pull the TP-40 sensors out and put in TP-10 sensors -- I don't know. Most Fatar keyswitches are the same (two long circuit boards), but I wouldn't say 'all', because I have only been inside a few versions of Fatar actions. There's also the fact that if you were to get a new action from Fatar by some miracle, it would cost several hundred dollars. Unless you are completely in love with your PC-88, it wouldn't make sense economically. I know how you feel though -- I will one day face the same dilemma with my beloved K2600X. I'll probably save for whatever Kurzweil's best workstation is at the time, but I will still hate parting with it. Especially if it's just the keys that are worn out and otherwise looks/works like new (as in its present state).
"Key assembly" or "Key bed" generally describes the entire assembly -- keys, hammers, sensors and all. Note that there is no "hammer assembly". They are all independent hammers which attach to the key frame (which is essentially the key bed) using a metal rod which passes through their fulcrums. It's kind of like a Rubics cube in that you can't just take one out [or all in one piece] and that they are not a system separate from the keys. Once they're installed, they're integrated. The whole system has to come apart piece-by-piece and then what you have is a pile of keys, a pile of hammers, a pile of tension springs, and then this long metal frame which they all attached to. If you order a "counterweight set" (good luck on that too
) you will likely get a set of 88 counterweights, which are basically these plastic blocks with metal inside that are shaped kind of like Tennessee. They have felt on one side and a hole near one end -- really pretty simple. Heck you could probably have some milled out of aluminum. It'd change the feel (maybe for the better?), but they would last for-freakin-ever! Really the individual hammers and a thin metal rod is all that there is to the weighted hammer mechanism. No 'assembly' per se, as they become part of the final key assembly/key frame/key bed/whatchacallit.
Hope this helps. Sorry for all the over-descriptive writing. If you have any specific questions or descriptions, let me know!
As far as I know, the TP-40 IS better -- better feel and no reliability issues that I've heard of. Second-hand knowledge of course.