Dr_Ugi:
It would invert your signal but I'm not sure it would matter.
Actually, the polarity doesn't seem to matter. It's the time between polarity changes that matters.
I think any op-amp would work. Connect the read head to the + and - inputs of the amp. That will turn all pulses into the same polarity and amplify them. Hook the output to an Arduino interrupt pin (2 or 3) and trigger on the rising edge to detect polarity changes. Long times are 0's and two short times are 1's.