Mounting a magnet on the wheel rim can be done but is not advisable,
for one thing you must get the wheel re-balanced profesionaly.I have seen it done on a couple of custom builds before but it has never looked good.
I would suggest using an optical shaft sensor on the end of the existing speedo drive cable.But if you plan to elimiate the speedo cable....
If the motorcycle is fitted with a drum style brake you might get away with mounting the sensor in there, or fix your magnet to the section of the disc brake nearest the axle , less stress due to reduced angular acceleration ,and dont forget the balance issue,you might need more than one identical mass magnet, and have the sensor on an a short arm from the caliper.
A good source for optical sensors are old mice, you may even find a use for the slotted discs :wink: