Where what you like. A kilt is just a garment, and tartan just a design. Besides, this is a free country. There are no tartan police, and there is even a sticky to let everyone know this. There are no more clans, only clan societies. No one is going to recognize the tartan and ask you if you're their long lost cousin, or expect an explanation. Is it nice to have a kilt to which you have a connection? Yes. Is it nessecary? No.

You should, however, be prepared for lots of questions like; Are you Scottish?, Do you play the bagpipes?, Are you Irish?(?), What are you wearing underneath your kilt?.