I have been doing an sr20 N/A conversion in my 1200 ute. As far as legalities go I think you can only go to 1800 forced induction but 2 ltr N/A is ok. from what I understand the ca18 engine is starting to get a bit old and not as an advanced engine as the sr so that is the reason I went for the sr option. it has been a huge job and has cost a fair bit so far.
Good luck and make sure you do a heap of reasearch first