You do not need to change the manifolds to run de-cats or HFC's.
If you do change the manifolds it's a PITA and the best option is to take the engine out, on an N/A Zed most don't think it is worth the effort of changing the manifolds - if you do, then go for something of a good quality and that has been tried before - Stillen would be my preferred option
Loads of advice on here about CAT's Zorsts etc have a read around the genral rule is buy cheap - buy twice