allo mate both of them engine conversions are piss. my mate had an xr2i 1.8 which alredy had a zetec, as dones a fiesta si. he put a 130 bhp 1.8 in it blow that up and now has a 2 litre. there are just stupid liitle things along the way that are different. such as the sump and oil pick up pipe in the sump. clutch and a couple of other little things. the best thing to do is take your si engine out, then get the 2.0 or 1.8 and just swap the bits over. personally i wouldn't bother with a 1.8 just go to a 2 litre. good luck mate.