While seemingly odd an inquiry to make, many ask if the long stem pushbutton from Suzo Happ or IL will fit into the low-profile Hori RAP series Fightstick.
Not Enough Clearance
The HRAP v4 Kai does not have enough clearance to accomodate the pushbutton and microswitch. The pushbutton is over 60mm long, more than triple the size of the existing low-profile pushbutton in the Fightstick. It is also thinner at 28mm, and requires a screw ring to keep from wobbling. The RAP cannot manage this ring in the limited space it has for snap buttons.
Short stem pushbuttons don't fare better, as you must still factor in the .187" terminal microswitch that is positioned below the housing, as opposed to inside the housing - a characteristic of most 30mm Japanese snapbuttons.