I have an old speedometer with the sensor on the fork but the part (magnet? piece or steel?) that goes on the spoke is missing.

I also have a part for the spoke from a different bike.

What are the chances that I'll get correct speedometer readings if I install the spoke part?

I'm calling it "spoke part" because I don't know whatcha call it .. maybe sensor target.

And I don't know if it needs to be a certain size/strength of target, or conversely if the sensor simply detects a binary "the sensor target is passing" signal, and counts those to get RPM. Probably that...