I would check the mechanism that shifts the bendex inside the starter, sometimes they will drag and need freeing up, common problem with ford starters. If it still staying engaged its not the bendex but the drive for it.
take that one bolt off that holds the dust cover over the shifter mechanisim and see if it is free. It is engaged by the magnetic field created by the field poles inside the starter when it has voltage applied to it from the remote solenoid, when voltage is released it should snap back, (spring return) if it doesnt return quickly it will stay engaged in the flywheel.
Usually a few drop of oil on the shifter mechanism is all that's needed.
goodluck, Charles