elefher:
Maybe i have to choose a magnet sensor but who?
Non Oriented Hall Effect Speed Sensors - Motion Sensors Inc.
Something like this sensor?
The linked sensor is certainly a possibility and would work very well in this type of application. That is if you don't mind spending the money on it. I haven't priced any Hall Effect sensors of that grade recently, but it wouldn't surprise me if that sensor was over $50.00 a piece (especially for a low volume order). Of course it appears to be a very robust unit in protective packaging intended for industrial conditions. Therefore without extenuating circumstance (like a crash damaging the front-end of your motocycle), you'll probably never have to replace it.
However, there are more budget friendly options which would still be effective. For example, here's a Hall Effect Sensor that's $0.95 at Sparkfun, and you could get something like it for at least a little less through a major electronics distributor like Digikey, Mouser, or Newark. The obvious difference is how it's packaged, but this one would need a small magnet to work properly. Whereas the one you linked to might be sensitive enough to detect the spokes of the wheel moving past it without a a magnet. Another low cost option would be a reed switch (that one has an extra case to make it more durable).