This has been asked over 1000 times throughout the years, the general consensus is it'd be cheaper and easier to buy an actual cosworth. There are a few that pop up from time to time on eBay that are already legal and in the US.
If you still want to do it, you would still need a donor cosworth for the conversion. Literally nothing is interchangeable. Completely different chassis and everything would have to be custom fabricated.
Now, you could do something like get one of these... http://en.wikipedia.org/wiki/Merkur_XR4Ti
And see if you could get a cosworth head mated to it's pinto block (which the cossie engine was based on).... which might cost you less than $3000 all in, depending how lucky you get, and how wrench smart you are.
But, if you want cossie-like power in a 2nd gen NA escort, you will want to think about options like KLZE motors from Japan or Focus ST motors from Europe.... or just building a solid turbo conversion for the BP motor.