Roush Saleen Body Kit and '01 Cobra

They are supposed to be direct swap. Make sure you spend more money on prep and fitment of the parts than the cost of painting.

From what I hear from the authentic saleen owners, they don't like seeing a fake saleen, but if you do... don't badge it a saleen.

admdavs,

We wont rip you a new one. To answer your question all you have to do is swap both bumpers, side skirts and add the wing of your choice (S281 or S351). Oh yea, maybe a hood too?
Save everything you swap so that you can bring your Cobra back stock if you ever sell. You can probably get away with just doing the front and rear bumpers, wing and side scoops and leave the side moldings (under the doors) and the c-pillar treatment alone on your Cobra. That will probably get the look you want.
You will save money if you buy from E-bay, just make sure it is a "real" Saleen part if that is your desire.
FriedChicken made a very good point. Make sure your prep and fit is perfect. Saleen body parts are a little difficult to work with to get right but if you take your time on the pre assembly, you will be happy you did.

I've seen some nice add-on kits (probably by buying the parts directly from saleen) and some very ugle ones. The kit has bad finish, doesn't fit right and so on...
And make sure you get the 18" wheels with it, otherwise it looks wierd, like fake, even with the 17"...
Another issue is your engine power. You don't want such a kit on a V6 and get smoked by a Civic...