handling.cfg is ignored by MTA and replaced by the setVehicleHandling function.
Looking at the web site you linked to, the important changes for the hydra seem to be:
setVehicleHandling( vehicle, "dragCoeff", 2.5 )
setVehicleHandling( vehicle, "maxVelocity", 799.9 )
setVehicleHandling( vehicle, "engineAcceleration", 35 )