Freeing up a rotary pump that seized and getting it to work again is just about impossible without replaced the head & rotor assembly. The tolerances in the distributor section of the head & rotor are extemely close. A few little nicks and scratches and it's all done and no good.
New repair parts have been unavailable for 20 years. Only fix is - to get a used pump like your's - or just convert your engine to use the JDB pump.
570 and 570A can use . . .
These CD pumps . . .CDC633-1AL, CDC633-18AL, CDC633-24AL
These CB pumps . . . CBC633-11AL, CBC633-28AL
These JDB pumps . . . JDB633AL2440, JDB633AL2486, JDB635AL2558, and JDB635MD2804