Engine Swaps!
When it comes to engine swaps the first limitation is your own abilities. Swapping to an engine from higher up the range such as an RS turbo swap is going to be a lot easier than fitting an engine that never saw service in these Escorts. Fitting an engine from a different manufacturer carries with it even more obstacles to overcome but it is not unachievable. I know of one Escort that was successfully converted to Rover K-Series power!
The easiest and most cost effective engine swap for something that was never fitted to the Escort is probably the Ford Zetec from the later Escorts and the Mondeos.