Hi Amun,
I had a similar problem last year and found a perfect solution for it:
the Behringer U-Control UMX61 is an inexpensive yet very good USB midi master keyboard with perfect
(and even adjustible) velocity sensitivity.
I bought such a thing secondhand for some 30 dollar, removed the casing and removed the keys, only keeping the complete electronics with the interconnectig wiring.
Then a miracle showed: the printed circuit board with the keying contacts fits the Fatar keyboard for 99,5 %.
Removing one notch from the Fatar makes the fit 100 %.
(I strongly suspect the Behringer keys are made by Fatar).
So in fact I changed the Fatar switches for the Behringer ones and that is it.
I kept all the surrounding electronics, put plastic sheets around it and left all that in my organ case.
Apart from having my Fatar keyboard with a very nice velocity sensitivity now there are these bonusses:
1) I don't need any midi circuitry anymore, as the Behringer electronics provide all that
2) There are many knobs and sliders on the Behringer electronics that you can use in Hauptwerk by autodetect (with the exception of the pitchbend wheel).
As soon as I found this out I bought another two Behringers and my Hauptwerk organ now has velocity sensitivity on all three manuals.
The newer Behringer UMX610 is probably as usable as the UMX61, as it seems it only changed colors (red housing in stead of grey).
Have fun,
Jack.