Kos-Mos is right on the money there. Other than what he mentioned, the rest as compatible as any other Armalite AEG would be in terms of internals and externals. Some aftermarket parts fit great, some others may take slight modication, but that's the case with many guns and aftermarket parts, not just ICS.
My ICS is a home-built job from a used receiver that I got here on ASC, and I build it up from there. It has a DBoys CQB front end (tight fit, it fit), a Magpul MOE grip (needed light grinding with a dremel to fit), and a G&P crane stock (one of the non-compatible ones lol). Shoots straight and far, and the hopup is excellent. It's way more stable than any other AR hopup. Very happy with this gun.