It is indeed a digital type signal that pulses as the wheel revolves - these are always hall effect /inductively coupled and generate an emf (voltage) that is induced by the electrical field changing due to a magnet or metal object passing through the field.
Now these devices all have three wires - power, ground & the O/P which is the one that pulses. You can typically replace any sensor with one from another vendor as the electronics in the speedo has a buffer that normalizes the voltage required for the circuitry.
The trick to matching is not so much in the magnitude of the voltage of the pulse that is generated, but in equating how many pulses per revolution there are between OEM & the aftermarket device utilized.
Now most after-market speedo's all have built-in calibration circuits so you can accommodate different number of input pulses per rev but the OEM gauge is fixed, so it's important to know how many pulses are generated in a single revolution of the front wheel by the OEM sensor
Also recognize that if you install a sensor on the rear wheel drive, it will be a different calibration because one revolution of the back wheel is different distance to one of the front, due to the different tire sizes. However you can install one of the after-market calibration devices to accommodate that fairly small difference (however they won't accomodate a substantial difference in number of pulses, like 2x or greater, more like maybe 50% max)
If indeed the OEM front wheel sensor is 4 per rev, (I don't absolutely know this for sure) then you can use any number of available after-market sensors on the rear wheel and utilize 4 trigger magnets then add the cal device (like Speedohealer or YellowBox) to fine tune it.
The key question is if the OEM is really 4 pulses per rev!
Is there any way to tell from looking inside the device?
Ideally you stick an o'scope of the O/put & just see how many pulses you detect in a revolution of the wheel. I'm not sure if you'll be able to see it with just a digital meter but you could try.
If I could temporarily get hold of a digital speedo or OEMsender, I be happy to test it out for the greater good.