No idea about this bike in particular, but based on 'normal' techniques, run power directly to the fan motor input. (use a length of wire directly from the battery, ideally with an inline fuse)
If it works, it works, if not, you probably need a new fan. (though do check if the fan is earth switched, by also running from the motor earth to a known good earth point)
Then look at the wiring between the temp sensor and the fan motor...do a continuity test with a multi meter to make sure there is a direct connection and the wire from the switch to the motor isn't broken
Then test that the sensor (switch) has a live supply to it....if it doesn't, then even of the switch 'opens' when at temp, it won't be able to power the fan.
Then, if possible (and as I say, I have no idea if this is feasible with this bike) you need to look at the sensor itself...in other applications, you'd take out the sensor, plonk the end of it in a pot of water that's above the temperature that it's supposed to kick in at, and it should 'open', allowing the power to go to the fan....if there is no change of continuity at the switch when it's at a temp that it should 'open', then the switch/sensor is probably fubared.
Do it all step by step, use a bit of logic and it shouldn't be too hard to track down
