if you was gunna do it i would go for an after market management due to to time invovled in putting the old focus mk1 parts in its easier to go aftermarket the focus rs engine would fit on the 1600 mounts but the gearbox im not sure of and as the rs has a different box might need some mounts making up the main thing you would have to check is the shafts

give someone like pumaspeed a ring to get the info needed but be cheeky and do it yourself
just remember anything is possible with anuf time and money