My turbo'd civic
 

i have a 99 ek turbo'd and i have a full apexi N1 exhaust..i beleive its a 2.5inch piping maybe 3? id have to double check but anyways could i put on a stock si exhaust and have it be alot more quiet? now its a deep sound but i want to hear pretty much nothing

Putting a stock exhaust kit on will NOT be a good idea. Turbos do not like backpressure, so you really do want to keep the 2.5" piping.

If anything, chop off that horrid N1 canister and get a twin loop, or even an Apexi WS2 muffler. Yes, they are more restrictive than the N1, but better than having an OE exhaust.

could I just use a 2000 civic si muffler? or is there no way of it getting on the piping?

The stock exhaust is only like 1.5" or something, which means the stock si muffler will have piping that size. And stock mufflers are meant to be quiet, and to be quiet means chambers and restriction. Just get a new muffler like they said an Apexi or like a Flowmaster, needs to be FREE FLOWING for the turbo.

You can put it on there, but there is no point. You are only as free-flowing as your most restrictive piece. Putting the smaller exhaust piping on would not only lose power, but I would assume your EGTs would go up, and I would guess that the turbo would be working harder to spin as needed.

Stock exhaust is 1 7/8" to be exact. +1 about the restrictive-ness of the OE muffler.