You will need to pick up some mounts. I am currently doing my b18cr into my dx swap. Here's a list of stuff you will need.
99-00 Si ac bracket
99-00 si timing belt side mount
99-00 si rear mount (i think its the t bracket)
throttle cable (i hear you modify your stock one to fit but i dont know how)

Since you have an ex you an use your wiring harness.

I am probably leaving stuff off, but oh well.