been opened up awhile

Sounds like the motor might not be that bad, if you want to "tinker" with it may work.
I would scuff them with scotch bite soaked in oil, this would absorb, the rust and most of the residue. Change the pad a lot, and cut them down to smaller sizes.
You really need to figure out why the heads were removed in the first place, I don't think many people tear a good motor apart.