You really don't need to remove all the conduit to rewire it. If some of the conduit is plugged with the black--tar like substance, just heat it up while being tipped down, and the stuff will run out on the floor.
That black stuff was just generally used for a couple inches at each end of a pipe, so getting it out is usually not a problem. Heating usually loosens the wire coating too and allows the wires to be removed more easily.
If you haven't broken any conduit, you're in great shape, and if you have, it CAN be replaced. As far as the access cover, they were usually aluminum and were generally stuck and easily destroyed. They are easily obtainable.
I think yours can still be used with a bit of luck and I know where to send you for parts if you break something. Keep us posted, and we'll help you thru it!
