An Escort cos is basically a short wheelbase sapph,in fact if you lift the rear of the carpet on the earlier models you can see half a grommet hole outline where the sapphire floorpans were cut and shortened(the pressing tools were changed for the later cars)...
The floorpans were then built into a 2 door escort shell at Karman in Germany(Clever blokes who are good at small model run specials).
You are fairly right in saying only the roof is original as the quarters are different(longer and slightly taller)wings,etc,etc,....all you are left with from the standard escort shell is....Roof,Inner quarters,scuttle,and Front upper panel...

Hope this helps....