I think installing a cat wouldn't be all that dificult. If you designed the exhaust to allow the cat to be removed and reinstalled it shouldn't be too bad. I worked in a shop where we did a lot of Porsche work for PCA Club Racing. At one time we went through a phase where we were using "cat tips" basically it was a small cat fabricated into an exhaust tip used specifically to get a car like yours to pass emissions so it couls remain registered, even though it never saw street time. This worked ok with 911's because the exhaust was short enough that the cat could still heat up enough to be effective. Does your exhaust exit before the rear wheels? Maybe this'd work for you... Just a thought...