The original ROG Ally has separate SSD, thumbstick, fan and battery guides; its M.2 2230 SSD is a modular storage part.
What to check before buying
Controls
iFixit lists left and right thumbstick guides as well as trigger and button procedures. A drifting stick is a different component from the display.
Storage and cooling
The SSD can be replaced or upgraded, while the two fans have separate guides. Match RC71L rather than the later Ally X when selecting parts.