On the first third of the track as the rider enters it, it would trigger a motion sensor.
What would? The track? How can the track trigger a motion sensor? The track should be holding still, shouldn't it?
Don't use pronouns, like it, unless it is abundantly clear what the pronoun refers to.
As the rider exits the end of the first third it also triggers a motion sensor.
Once again, WHAT triggers the motion sensor?
The ONLY thing that can trigger the motion sensor is the rider. Knowing that there is a rider present will NOT tell you where the rider is along the track, unless the sensitivity, and placement of course, of the motion sensor is such that the sensor doesn't detect motion until the rider is along the portion of the track where the target should move.
The distance between the two motion sensor will be 33 yards.
Most PIR sensors don't have anything like that range.
So basically i need a system that can measure the riders speed using two motion sensor and using that readout
That is not going to happen. What readout? Humans read things displayed somewhere. You haven't proposed displaying anything for a human to read.
Some horses will go slow some will haul ass.
Shouldn't the rider be the one controlling the speed?
If you can assume that the horse and rider travel the 2nd 3rd of the track at relatively the same speed as they travel the 1st 3rd, you could use low power lasers with break beam sensors near the start of the track and near the end of the first third. The timing between breaking the two beams will give you an indication of the speed. Make the target rotate, assuming that the horse/rider speed will not change significantly.
If the speed does change significantly, tough shit. That rider should have a harder time hitting the target.