First off LOOSE the grease gun on the plastic shields!!!! All that does is holds the dirt in the shield.
Second all yours needs is cleaned up and put back together. All you need is a new inter plastic piece. They will not be much money. Maybe $5-10. Then clean off all the grease and Do not grease it again. The steel slides and the cross needs grease the plastic shield does not. Take the chains off the shield and let it rotate with the PTO shaft while it is running. Just make sure the shield will stop while the shaft is running. This way there is little wear on the shield as it is not stopped all of the time. Just when ever something rubs up against it.
As for running it without any shield. You are gambling with your life and anyone that is around that shaft. The complete new shield would not be over $50-75 at the most. I value my life and others too much to run ANYTHING without a PTO shield!!!!