You might check with the usual suspects (borla, magnaflow, petty motorsports, 42 draft designs, etc); most seem to have the 2.0T dual rear exit exhaust systems, i'm assuming the 1.8T is a single rear exit exhaust?
Good question; you probably need to research things and may need a new bumper from the 2.0T models, for the dual or modify what your current bumper you have? If the previous generation new beetles are any i indication of compatibility; many beetle, and golf exhaust systems were similar or need slight changes to work. I would contact the various exhaust system companies and see what they say; those around here, that upgraded their exhaust systems were 2.0T engines with pre existing dual exhausts (cat back). You might pose this question, on the vwvortex beetle forum; as many owners, have heavilly modded their Beetles there and someone may have done this to the non 2.0T Beetles.
What are you looking for?
More noise? More power? Both?
I cant personally vouch for any fitment issues between the 2.0 and the 1.8, but I can say that EVERY aftermarket company out there does not think highly of the beetle.
There ARE some that actually care to do some Research and development, but on a personal point, I can't TELL you how much money I"ve spent TRYING to make stuff off a GOlF, or GTI, or passat, or jetta work, just to SEE if it would.