Hi
f you have the OEM sensor then this looks (although I have never disassembled one) like a field modulation device that is converted into a speed measurement. I think you would need to replacement with one tha offers the same characteristics Is the R1 wheel axle the dame diameter as the FZS as this might suggest the OEM sensor would fit - I'm not a mechanical expert so maybe talking poo here?? From the sensor electrical side, air gaps in the wheel/teeth/sensor also play a part as does operating temperature, the comparator and amplification stages. I dont know if the FZS does all this and how as I've never studied this in detail (taken it all apart and investigated/explored)