I have raised this issue at the FT forum and especially for the Refill ball-bearing valve I never got an answer. The issue became apparent through a group order of the Hcigar version in Australia. All the screws are replaceable by the user but the ball-bearing valve (+spring) is not. Hcigar replaced the bases for all (in the group order) because of the valve.
I am expecting an Hcigar one from the US and according to the vendor who communicated with Hcigar everything is 100% SS now (incl the valve + spring).
As far as the fasttech version is concerned I wouldn't bet my money on it unless I got a definite answer from someone there or through a test.
It is easy to test by soaking the atomizer in water for a few hours or overnight. If the valve parts and the screws are not SS the difference in metals will cause galvanic corrosion through the water which acts as an electrolyte and rust will appear. Our liquids are worse electrolytes than water and slower to act bu it will happen eventually.
Right now I would only get the lite from there which I just received recently and it is a Tobeco clone. The threads are not the best to be honest and compared to every original atomizer I have they are really rough. E.g. compared to my DID & Gemini from MMV which are extremely smooth and high quality.
I have ordered 2 original replacement kits for the lite clone from fastech (incl the o-rings) just to be sure. However, as I said above you can't replace the ball-bearing valve with the spring on the 3.1 so if you get it you gamble.
All in all, if the Kayfun suits my taste I will probably get an original unless the Aqua proves to better (will also get a clone to try).