what engine have you got?
also mk3 si's came with power steering so (depending on what engine) would be a straight swap for its steering set up.
not sure but i yhink ive seen it on 1.1-1.3 too not sure tho.

1-power rack
2-power steering bottle with all relevant pipes
3-power steering pump
i think you may need to change the steering coloum that connects to the rack as im not sure if its diffrent on power to manual.
also when you put it on if you ever change your lower arms DONT tell them its got power steering because there is TWO diffrent ball joint sizes one for manual 1 for power.
hope this helps
no doubt someone will correct me on the coloum.lol