How is the SP400 calibrated?
In these days of digital signal processing, you don't really need a calibration process.
The VOR or ILS signal is demodulated to just the components which are all in the audio spectrum. This is called a "composite" signal (erroneously, many avionics people call it a "composite video" signal, which is something completely different). Then, elementary DSP techniques can be used to generate a CDI or RMI presentation from this, and you can do handy things like suppressing GS indications in the absence of a valid LOC, or suppressing GS if the thing thinks you are flying a BC approach.
In the past, you had a circuit board full of trimpots (e.g. the notoriously unreliable
KN72 or some other "box" to which pilots would routinely trust their lives because it could fail in subtle ways) but today this would be just some firmware.
It would be trivial to test the SP-400 with a common avionics tester like the IFR-4000, or any of the cheaper boxes in common use.
There again, I would not be using a $399 handheld for a precision approach! I'd be making for my alternate!
Not if your panel is dead.