There's no such thing as "FAA Airways".
An "IFR certification" on t'other side of the pond generally refers to the pitot-static check status; two yearly, ISTR.
In FAA land an aircraft must carry avionics and instruments suitable for the planned route.........so if you are over here and want to fly "airways" then you require the CAA-specified fit. Hence the probs with some of the new new plastic-fantastics because they don't carry an ADF as standard - in the US it's not required.