All rst's are 1.6. I think the last frst was on a j plate. Both escort and fiesta rst use the same engine so one isnt any more of a straight fit than the other. The engine fitment wont be the problem, the wiring of the frst may be as it efi. It has a few extra sensors and a different inlet manifold. Apart from that they should be identical.
Come to think of it the frst may have a smaller turbo too.