If the header set up is such that the heater box bolts to a pipe then is fed into the CAT then the muffler, then you could use a Dynomax Super Turbo muffler.  18" long 6" round canister.  Cost is @58.  Should fit in stock location with the arms needing to be bent down @1" to hold muffler.

I am running one on my modified 2.1L WBX and it is quiet, even at 85 mph(Ask Ken lewis <kdlewis@JUNO.COM> He went for a ride.)  You would need to get a three bolt flange (the kit comes with a two bolt).

Robert