My G50 came with the standard speed sensor still installed. I used VDO "Cockpit Vision" instruments, & the speedo is magic - you just set it up in cal mode, drive 1 measured km, then hit the set button - everything is automatically calibrated & it just doesn't care how many pulses per rev it gets ! It turned out to be accurate within 2kmph at 100kmph when measured against Trevor's calibrated system.
The VDO can handle "Hall Effect", "Interruption Oscillator", or "Induction" sensors - just plug the sensor into the right set of terminals on the back of the speedo. I can't be 100% sure (too long ago), but I think that the G50 sensor was an induction type as per Bill B's comments.
I have no idea of the number of pusles/rev, but Autohaus (Posche g/box experts in the US) would certainly know - they are very helpful guys, even if they don't get to sell you their staggeringly expensive gear sets !
Kind Regards,
Peter D.