These cars were created to meet homologation regs to enter group A rallying, why would they use an old engine and a different turbo not fit for this?
I think this is probably a story that has got confused over the years, I am sure some of the original old mules from around 90/91 maybe even before would have used Cosworth engines from what ever was available at the time (shoehorned into a mk IV RS Turbo shell anyone). These were never sold though and Ford would have had no intention to do so.