My OptoKey just sends and receives the 44 notes of the keyboard itself.
It doesn't receive or transmit any modulation or pitchbend, unfortunately.
However it does replace the entire contact board in a Mini and has very fast response.
This is good because when one removes their original contact board, they're also removing some spring tension from the keys.
The keyboard then becomes much easier to play fast.
As for checking out a Mini, I'd suggest a few tests:
1. Place the pitch wheel in the center and perfectly tune one VCO against the A-440 signal.
Then move the wheel some and place it again in the center.
If the Mini is no longer exactly in tune with the A-440, the pot may be bad.
2. Hold a high note on the keyboard, then play notes below it.
This can help indicate if the key contacts are dirty. (dirty contacts will "glitch")
3. Tune all three VCOs to the same pitch, then change the octave selectors one by one.
If they do not perfectly land on octave intervals, the instrument may need calibration.
If your Mini was an early unit, it could also be that it lacked a later update known as a buffer board.
But Zombie's Mini appears to be a more recent model which came with a buffer board already installed.
4. Check all switches for glitching or noise.
The switches most common to have problems are the two orange switches on the left and the noise's pink/white switch.
The A-440 and main output switch can also make noise when switching, but usually it's normal.
5. It's normal for the overload lamp to flash when the instrument is turned on, but it should not stay on.
6. Verify all pots for any noise when turning.
7. Check all keys for physical noise (clacking when pressed or quickly released.)
There are more tests one could perform, but this can give a rough idea what condition a Mini is in.
This particular Mini appears to be a later model and this is a good thing in regards to wear.
Fwiw, this below is a sheet I've used to evaluate Minis that arrive here for service.
It's not terribly gorgeous, but just helps document problems.

Better to be king for a night than schmuck for a lifetime. - R. Pupkin