Very difficult, someone was asking a question the other day as they never had Cosworth's where he was located and he wanted to convert his, there are so many differences between the two.

The engine is the simple part, you then have suspension, brakes, driveshafts and propshaft are different etc, you then need to source a gearbox and if going with the original setup of a 2WD, you will need a completly different rear beam, along with different diff, both of which are unique

You would really need a donor Cosworth, you can of course go to the effort of buying all the parts, but it won't be cheap and will be time consuming, hence why it's easier just to buy one outright!

Martin