The ring gear is normally a pressure fit onto the flywheel ie two different types of metal the ring is heated put on the flywheel and when it cools its immposible to move replacement was done the opposite way round.I have never heard of one moving but there is always a first time.The fact that it has moved should not stop it starting as the ring gear is the same 360 degrees it was common practice to move the whole flywheel round 90 degrees to an unworn section a flywheel will always stops in the same place therefore wear in the same place. You need to remove the starter and make sure the starter cog is flying forward on the spindle and there is no wear.I would think that the problem is wear on the starter but if the ring gear has moved backwards as opposed to rotary then its in danger of falling off and wrecking the whole engine and gearbox the solution maybe a new flywheel with ring gear .I would be interested in what you find.